I have a fair few bits and pieces of feedback to share in the coming days, but thought the most fun and interesting place to start would be with the Summoners themselves. As for my bona fides, I currently have 57 hours played, have beaten the game with most Summoners on Hardcore - in fact I exclusively play Hardcore, and I would have had a full collection if I liked Celestial a bit more.
I am going to be using the slangterm "voltron" to describe a certain playstyle centered around buffing your Summoner. I assume most people around these parts would be familiar with it from Magic The Gathering's EDH format, but better safe than sorry.
Antarax: Super cool Summoner who manages to recontextualise a ton of cards. I love how he makes you initially think you should focus on Skeleton and Undead synergies and then eventually reveals himself to be a voltron beast thanks to his obscenely powerful Lifesteal and Pierce traits. However, I particularly fancy how he manages to sell the player on the fantasy of being a necromancer, as you gain more "bodies" (undead minions) by effectively reanimating slain enemies. wonderful bit of ludonarration there.
Antarax does face a small issue, and this is one that a fair few Summoners struggle with, in that he really wants a certain type of Guardians, namely Undead Guardians, so that he won't cripple his own team with spells like Plague or Poison Cloud, and it can be surprisingly difficult to find the Guardians that you want and/or need with the current way Guardian aquisition is handled.
In fact, and here's a point that'll be repeated often in these feedback threads: The dungeon biome is by far the most fun one to begin in, and a huge part of that is that you are guaranteed at least 2 Guardians without having to roll any dice. Heavens do I hate rolling dice for my initial two Guardians...
Sibylla: My personal favorite character. I've always had a thing for sacrifice themes in card games and absolutely love the things she can do with a Phoenix in hand. Tons of fun.
However, I feel like her +1/+1 undermines her sacrifice theme in favour of plain minion-spam and I have even found that the extra health has an unfortunate tendency to occassionally mess you up. I’m going to suggest changing her passive boost to +1/+0. This way you are still incentivising the player to play minions aggressively and to also treat their minions as expendable sacrificial fodder. Similarly I feel like healing for 3 isn't exactly the most impactful way to signpost Sibylla's sacrifice/swarm theme to the player. Maybe if it was, say, heal 3 and gain 1 mana, or something like that? Something to really let the player know that on-demand minion deaths are powerful and potent.
Sibylla also stands out as being quite possibly the character who is hurt the most by how the game handles distributes cards depending on one's Summoner class, that and D'ork. Half of the most fun and interesting spells that match Sibylla's profile (Phoenix, Magma Golem, etc) are in Chaos, but because she is main-Void she will only get offered a vanishingly small amount of Chaos cards throughout a run. In fact, my general go-to for starter decks, no matter the Summoner, is to never start with cards, even potential must-haves, from my main-class simply because of how much more common main-class cards are.
Antarus: My secondmost favorite Summoner. His "combo" theme is a lot of fun and forces the player to think about the battlefield layout, which can lead to some really interesting situations and play-patterns. Unfortunately the entire forest biome is currently devoid of the battlefield hazzards that Antarus wants to manipulate and take advantage of. In fact, Antarus might just be the character where the biome affects the amount of fun you get to have the most. The dungeon biome is amazingly fun (as it is for everyone. Best biome by far) thanks to all the traps, the desert biome is decently fun - if a bit cumbersome at times, and the forest biome just flat out refuses to let Antarus do anything without setup.
I also feel like Antarus might be the character who is treated the worst by his suggested starting deck, as it doesn't feature any of the cards that I'd call must-haves for it. Things like Seeker Barrage, Lightning Storm, Fire Trap, Wall of Fire, or Dragon Hatcher. Admittedly this might be in order to let the player discover that Antarus, but his starting setup really does him no favours.
Late recommendation for something that I feel like Antarus honestly lacks: Access to a minion with Collide. Knockback is great and all, but sometimes what you really want is something to knock things into.
D’orc Slayer: He currently feels a bit all over the place as he wants to have a Goblins/Orc theme, and he wants to go for a Voltron theme AND he wants to go for an “injured” theme. The latter of which is in rather dire need of more support. He similarly is the Summoner who is the most desperate to get specific Guardians, namely the Orc Shaman and potentially the Perserver, in order to get the ball rolling. His goblin-summoning being quite similar too Sylvia's squirrel-summoning does not help sweeten the deal either.
On top of everything else "orc warchief" is, well, rather generic and doesn't make all that terribly much sense as a theme for a magical spellslinger chosen by destiny itself. In fact, I would strongly recommend not using him for the game's promotional material. The imagery of an orc and a human charging at one another is, as you might have noticed, quite overdone and does not exactly help catch people's interest.
Overall I have to say that he might just be my least favorite Summoner and the one who manages to stand out both the most and the least. The most because of how many issues he has (Orcs and Goblins largely being Free-Will making it difficult to collect them, Orc Battlehorn being one of the few "forced" cards that cannot reasonably be used by anybody else, etc) and least because of how unremarkable much of his toolkit is.
Once that has been said, the "injured"-theme is a ton of fun and has a lot of potential. Even if the Enraged spell is one of the incredibly few cards in the game that are just outright bad and the rewards and enablers for the theme aren't quite "there".
Li: Incredibly generic abilities, which isn’t a bad thing, but other than Fire Elemental and Fire Shield she doesn’t really have any build-arounds. She also only has few tools in Celestial, namely Time Bomb and Smite. I’m going to suggest adding some fire vulnerability into the game to give Li a reason to care about fire damage and I am additionally going to recommend making a minion or two that are immune to damage either caused by their allies or cannot take damage during their master’s turn.
Uhh, uhh, a spell that gains extra benefits from Spellpower, or a minion that wants you to have Spellpower could also be really neat. Helps Li out without having something that's borderline unplayable by everybody else (looking at you Orc Champion and Orc Battlehorn...).
I quite like her, but there really isn't a lot to be said about her. She's just kind of... there currently, although her issues are largely down to the current cardpool. She doesn't have much going for her at the moment, so unless you luck out and get a ton of crowd control spells (Giant Meteor, Ice Shards, Three Point Palm, Hyperspeed) and some really good Guardians you'll quickly find yourself overwhelmed in Hardcore mode. My Li run was the most harrowing one so far, and it was almost lost on numerous occassions.
Cora: Ice Dweller is not exactly the most elegant name. “Cold”, maybe? Overall a super powerful, if somewhat liniar, character with two build-around cards, one of which is the incredibly powerful Shatter. Seriously, it is never not the correct choice to start with two copies of Shatter - it is easily her best card. She might actually be a bit too good. The slapped on ice vulnerability that minoins get after having been frozen comes kind of out of nowhere and feels a bit overkill. In fact, it feels very overkill.
I believe it bears to mention here that, much like goblin/orcs, summoning people in general will always feel somewhat off and weird. It does not matter what the armour of a dude looks like, what abilities he has, or what his culture is, summoning a dude is summoning a dude and summoning a dude creates a scenario in which you have to ponder why you aren't summoning something that it makes thematical sense to summon like, say, a demon or an elemental instead. Do... do the bandits that I summon require pay? Do they need food?
Zenon: Having absolutely no reason to ever use Celestial cards, ever, effectively makes his secondary class useless and an active detriment. It also gives him by far the most obvious path for deckbuilding, as there really is no reason to just fill your deck with mana-generation and card draw so that you can get Power Overwhelming going as soon as possible. Consider either changing his ability into caring about non-minion/non-equipment cards instead, changing the ability entirely or making him a pure Arcane character.
Sylvia: Pied Piper currently being obviously overpowered aside, I actually quite like Sylvia. She has numerous routes she can go down (Small animals, animals Insectiod, buffs, nukes and crowd control, Poison) while still stearing you in a very clear direction (small animals) and she is currently the only Summoner who can really take advantage of the full list of Satchel Cards. Sure, she might want a few more incentives to go dapple in Chaos magic, but overal she is great fun and is likely to be even more fun once Pied Piber is tuned down a smidgen.
Albirich: Perfectly fine and functional. Wish Celestial magic felt more… magical, but that’s the tradeoff for having summonable humans... I guess. I believe it does bear to stress that Albirich is the character who made me realise just how obscenely powerful anything with "moves 6 spaces" is. In fact, "moves 6 spaces" is so obscenely powerful that I am going to strongly recommend adding a 3rd stat to every Minion, Summoner and Guardian to denote mobility, so as to allow for movement to exist on a scale between 1-6 as a balancing factor rather than have 3 be the universal norm and a whooping 6 be the one exeption to the rule.
Similarly Albirich feels a bit undertuned compared to Antarax, but I suspect that is largely down to Antarax' Lifesteal letting him go absolutely haywire on most enemies after a single Bloodlust, whereas Albirich needs a bit more support first. I also have to say that I quite like how well Albirich functions with Small Animals and that I really do wish the player had a bit more control over what class of cards they get so that Small Animals Albirich (and other themes that rely on a Summoner's secondary class) would be more viable.
EDIT: I thought listing the starter decks I tend to run with for each of them might be useful. Notice that I never, ever, pick a card from the main class if I can avoid it, exception being Zenon, because of how much rarer secondary class and neutral cards tend to be:
Antarax: Bloodlust x2, Teleport.
Sibylla: Magma Golem x2, Phoenix.
Antarus: Seeker Barrage x2, Fire Trap (begrudingly)
D'orc: Orc Battlehorn x2, Orc Champion/Goblin Champion
Li: Time Bomb x2, Smite/Wind Elemental
Albirich: Savage Transformation x2, Fennec Fox/Unusual Size
Cora: Shatter x2, Frozen Blades
Zenon: Largely whatever. Iron Golems and Seeker Barrages and stuff. :V
Sylvia: Detonate 2x, Hellhound