u/AnamorphicWeirdWill2

1. The very obvious one: Lag. We all know how bad some matches are, though it's been worse in the past. The most they can do is clever matchmaking and optimizing data transmission. But if some people have high-latency connections there's only so much you can fix. You can't force people to not play PvP at all if they don't have good connections (barring lag-switching of course).
2. The game sometimes hiccups and doesn't register the commands or doesn't execute them in the intended order. This is often (but not always!) a consequence of point 1, but damn it's far more common than it should be and it's really annoying at times. It makes you do (or not do) things, in a way that instantly loses you the match. So many inconsistently dropped-combos, too... A related addendum would be the fact that we still can't un-queue actions, something I think would be a really nice update, though this is more subjective.
3. The buffs line is still a bit confusing. Though they did improvements such as adding things like the anti-cover icon on characters, they still need separate or smarter ways to show some other specific buffs such as endurance penetration instead of them being buried by the 5543 damage stacks modern units get for existing.
4. Perfect Vanishes. Look, I don't want PVs to get fully removed or anything, and they did at least make it so that you can't PV long range blasts... but holy hell are PVs still stupid, random and chaotic. ONE SECOND into the match and you can get a random PV because the opponent dared to initiate. I dare to do so much as FART and the opponent somehow gets a PV. You are last man standing against 3 full HP units, the opponent doesn't do anything except preventing you from overcharging, you dare to use a card, they get a PV and you lost. Only maybe 10-20% of PVs are a result of skillful predictions and/or the opponent doing objectively stupid plays. You get the point. They should make it impossible for the FIRST action of a player in a match to be a PV. Prevent players from getting multiple PVs in a short amount of time and/or make them more rare, maybe even dynamically change the timings for extreme scenarios (such as 1V3s) though I guess this may feel too weird and offputting.
5. Quick attack usage/matching is inconsistent. How often you KNOW the opponent is going for a quick attack after a combo, you TRY to match it but your unit just does the fuckass tap blast and you die? Or maybe YOU are trying to do a quick attack after a combo.. and instead it's... the fuckass tap blast. Or maybe you did a quick dash and wanted to continue with a card... that the game conveniently ignores. I guess lag has a role again, but come on...
6. There needs to be some mis-tap rejection in the "card zone". How often enemy units randomly destroy/lock your card JUST as you're about to press it, and it either turns into a tap/tackle or you use a completely different card you drew in that exact moment, resulting in a drop and/or you getting RRd/Overdrived? You can't do anything about it. It's another insta-loss in many scenarios.
7. WHY. IS. IT. SO. STUPIDLY. HARD. TO. MOVE. FORWARD. TO. CLOSE. RANGE. WITH. YOUR. UNIT?!??!?!?!?!?!?!!!!!!!!!!!!!?????????!?!?!?! HOLY HELL this is probably the thing that is most infuriating for me personally, so many times I tried to move up from mid-range, the game doesn't care and I end up doing a mid range strike or tackle. WHY. It feels like you have to go through a liquid concrete layer or something. But it's so inconsistent, sometimes it just works, sometimes it takes too much and you're stuck at mid-range, or you have to take a second too much to actually move up and you lose your timing (the opponent has far too much time to see and react). This is almost never the case when moving from full range to mid range. Even going backwards from short range to mid range is not particularly hard (unless there's a lot of lag).

If you feel there's something I missed, feel free to add your own