The biggest problem that I've had with Illyia so far is momentum. The game is not easy, and that is quite all right. However, I've found it exceptionally difficult to get into a rhythm or a groove playing it, much to my chagrin. Here's a few reasons I think why that is.
Momentum (but like the movement kind)
Illyia's movement feels like it's almost great. The triple jump feels great, sliding is a ton of fun, and if you can combine those with pogos you feel like a parkour master. However.
If you're on the ground and moving around, swinging stops you in place, killing your momentum. If you miss your timing on a triple jump, Illyia rolls and you lose your momentum. Illyia feels like she can bounce around like a rubber ball, but if you mess up slightly, she goes from rubber to a brick, and that never feels good. But the movement leads into the combat, and there you have two types of momentum to deal with.
Combat Momentum (and getting Spelunkied)
I love Spelunky. Absolute bastard of a game, but feels great to get a good run in. Part of the reason it's a bastard is because of the Series of Unfortunate Events that occurs when you get hit. You're often knocked back and stunned until you've touched the ground for long enough, but you aren't invincible during this duration, have zero control, and if something else hits you, you get sent for another ride. Combined with the gratuitous amount of cliffs and low amount of hits you can take, this means getting touched once can at any point be a cutscene leading to your death. I gotta explain how this is relevant.
Illyia stops moving if she swings on the ground. Alongside the movement momentum, this kind of means that if you're not doing roll/counterattack on the ground, you're locking yourself in place to get hit, since sword attacks don't seem to stun or knock back enemies (except on energy strike or the last hit of a combo). This means it's generally easier and/or safer to be pogoing from above, in the air, where some enemies can't do anything about it. And that feels right, with how good the movement can be. It feels right to be in the air. But, if you get hit out of the air, or you just happen to be near a ledge, you can get Spelunkied- you're sent flying, and whether you hit the second wind tech or not, you are out of the fight for a bit.
Momentum is poorly defined in fighting games (google doesn't help), but there's a phenomenon where the person winning, or hitting the other person, is more likely to continue winning. The person controlling the pace of the combat is controlling the momentum, and in addition, there's some amount of mental game that goes into that- getting hit sucks, and hitting the other person feels good. Getting into your rhythm, your flow, and denying your opponent theirs, means you're gonna do much better.
This also applies to games like Hollow Knight or Illyia. Getting hit in Hollow Knight, you get knocked back a tiny bit and have to readjust for a moment. It's a minor setback, but if you're in the groove, you can stay in the groove- a hit isn't a big deal, you can keep your momentum unless you hit spikes. But given how far Illyia gets flung, and the fact that you have to pause and focus on hitting your second wind (or risk getting hit by contact damage or another enemy), you have to stop fighting to re-center yourself. Your momentum is dead on impact.
That sucks. The combat is otherwise fun, and so it sucks to be taken out of it when you make a mistake.
Illyia's never been locked into it