I got my last dude (Oldur) to 6 last night. I'm still going to play everyone until I get them all to 10 (because of course) but I'm feeling like I have a pretty good grasp on the characters right now.
I'm not sure I can tier them by power level because I can't play all of them at that level. What I can do is rate them more or less based on straightforwardness to play "correctly", difficulty of execution, and number of things you need to juggle. I'll try to order the characters within categories by approximate effectiveness, though.
Ranged is in my opinion the simplest archetype to play. There's slightly less to keep track of and things are a bit less chaotic. You usually have a bit more time to respond to most potential hazards. Dealing with a melee who wants to kill you can be challenging though.
Ashka is probably the easiest ranged DPS to pick up and play, and he's strong. He's got a good long range poke, a good M2 against anyone who has no defensive ability, good mobility, and some good extra skills. His EXes are also great. At higher levels, he can be much more aggressive and output a lot of damage and CC.
Jade is not hard to learn, and relatively easy to be effective with. The round 1 rite to give her immaterial on stealth is very strong and instantly increases her survivability a lot. Her strategy is straightfoward.
Taya has the most damage output against a clumped, organized team of maybe anyone in the game. But she's pretty hard to learn, relatively speaking, because she doesn't have outs and her damage needs setup. She's kind of reliant on her teammate early in the round to help set her up. And she has to get in and mix it up against some enemy compositions.
Iva is kind of an odd duck for ranged characters. Her M1 is barely longer than melee range, but what she's got going for her is a selection of good support skills, and ridiculously high burst/sustain at close range. If you jump on someone who has no outs left, unload your shotgun and then flamethrower them, that's like 100 damage in 5 seconds or something. Her ranged spellblock into stun is good, and her ranged EX incap is amazing for setting up combos. And her ability to shield a teammate is pretty good, too.
Varesh is maybe the hardest ranged to play, in my opinion. I was able to play at-level with him without feeling like I was being particularly efficient. You have to juggle maintaining multiple debuffs on multiple people, consuming those debuffs for effect as often as reasonable, landing lots of hits and small AoEs. Varesh punishes you more than most for missing a hit and dropping your combo. But if you can do it, he's really, really strong.
In terms of tiers, I can't point to any and say "this one is significantly worse than the others." I think Ashka and Taya are "probably" better than Jade, and Varesh might be S-tier if anyone can actually reach his skill ceiling.
Melee are harder to play than ranged because you have to get in. This exposes you to taking a lot more burst, and can be hard in general. So it requires more positional and situational awareness, I think. You also have to know when to get out, and how to do that.
Bakko is I think the easiest melee to learn, and he feels pretty easy to be effective with, to me. But that's kind of subjective? He's got a solid toolset and a lot of panic buttons. He also has a hilariously good ranged poke on his M2. He can work in basically any team comp, too.
Freya is also pretty straightforward - get in, get shields, kill stuff. The execution can be a bit harder - she has fast animations and short cooldowns, and if she gets stuck in a bad situation it can be hard to get out. I didn't click with her, but I know she can be effective.
Shifu is also relatively straightfoward, but a bit harder to play well. He's really good at chasing, but it's also really easy to get tunnel vision and Just Die with him. He's got good survivability between his R1 healing rite, a counter that provides immaterial, and a space that provides immaterial. And his damage output is solid and sustainable, although his burst isn't that great. The biggest challenge with Shifu is I think working as part of a team instead of just doing your own thing and hoping it all works out.
Rook is very feast or famine (hah hah.) If you get in, he's great. If not, he's hard. Playing him well is non-obvious, because you have to be extremely patient and careful... or have a teammate set them up for you or help you get in. But his plan once he gets an opportunity is extremely straight-forward. Smash, eat, smash some more.
Croak is probably the hardest melee to play effectively once you get passed the level of opponents not knowing how to deal with Stealth. I see more Croaks being completely ineffectual than anyone else. He has a hard time getting in against careful players, and his sustained damage is pretty low. But as a disruption/burst melee, he's pretty strong. You should be using EX stealth whenever you can, and his EX M2 is pretty good also.
Overall, I think Bakko and Croak are probably the strongest melees. Bakko's toolkit is just so good and his burst is so high. Croak's EX Q can be a gamewinner if you get a single incap one, stun the other and your teammate helps with the burst. Freya and Shifu are solid, but I feel like Bakko and Croak are just a little bit better. Rook... I don't think is bad, but I think his lack of mobility can be a problem.
Worth noting is that Croak and Rook may be the most game-warping characters in terms of how opponents have to respond to their presence in the game.
Supports are the most complicated type to play, I think, because you have to have even more field awareness and make more complicated decisions.
Pearl is definitely the easiest support to play and the easiest to be effective with. All of her abilities are straightfoward and strong. Her heal can be a little hard to land, but there are no shocking complexities to her abilities, or weird interactions.
Poloma is probably the next easiest support to grok and play well. If you hold M1 all the time when you're not using other abilities, you'll do okay. There's some judgement on Otherside and how to use her space that can be complicated, and EX space is harder to know how to use correctly. But she's got a strong kit and can handle being focused without too much trouble - she arguably has 3 escapes.
I'd say Oldur is probably more straightfoward than Lucie or Sirius. He's got a "normal" set of skills. You M1 and M2 from range, more or less. The tricky bit about Oldur is recognizing that you're not going to really help keep your teammates alive as much as you're going to help your teammates kill people faster. If your teammate is a melee, you use your heal charges, bubble if they try to disengage, and otherwise just DPS your face off. You can do a lot of DPS. If your teammate is ranged, you can help peel for them with snare and protect them with bubble or interpose with normal shield. Oldur is a little squishy against melee pressure if he doesn't have energy, though.
Lucie is a bit harder to play. She needs good positional decision-making because she's not maneuverable at all. Ideally, you want your opponents to attack your teammate so you can shield and heal them. If you get focused, EX fear is great. Otherwise, her panic flask is great. She also needs to be careful with energy so she can always roll if she needs to, but also use EX M1 as much as possible for that sweet, sweet damage. Using Clarity effectively also takes a lot of game knowledge.
Sirius is maybe the hardest support to play, because he's not a ranged support like the rest. In order to play Sirius you have to get up and personal. So you've got the mental overhead of being a melee with the mental overhead of being a support. He also has no extra panic buttons on his EX or R abilities - if your space and Q are on cooldown, you're boned. But if you play him well, he can do oodles of control and protection and damage. His EXes give him pretty shocking burst. His EX space is one of those "you should almost always use the EX version if you have energy" abilities.
Overall, I think in 2s they're all pretty solid, although Oldur may be the weakest just because he has fewer control options, so if he can't deal damage he can't do much. In 3s, I think Oldur and Sirius may be approximately unplayable, and Poloma becomes S-tier because of the way her heal/damage sharing scales. Pearl is maybe a little strong and I think Poloma is probably going to get nerfed more (probably with Wolf damage reduction or cooldown increase, if nothing else.)
Cross-archetype, I think probably the hardest character in the game is Varesh, then Sirius, then Croak. The highest skill ceilings I think are Varesh, then all the supports, then maybe Croak and Freya, then Iva and Taya. But I don't know if peak performance will necessarily correlate to skill ceiling.
I don't really have any interest in trying to do "best" character comparisons or tier lists that try to go cross-archetype because I think those are kind of apples-to-oranges comparisons.
I think Poloma is "too strong" relative to other characters - she has the most heals of any support and very-high damage output. I expect Pearl to get slightly nerfed eventually, too. I wouldn't be surprised to see nerfs to Freya, Shifu, Bakko, Jade, or Ashka. I also except a buff or rebalance of Rook at some point. I wouldn't be surprised to see a small buff to Sirius or another small buff to Oldur. But I have pretty low confidence in most of this except the Poloma and Pearl nerfs and the Rook and Sirius buffs.