The first 2hrs or so are straight up garbage. It's a bunch of rehashed fights we've already done, and some cutscene changes or expansion. The expansions are great, the rest is not.
You can choose who to play as in some fights (Roxas, Kairi, Aqua, Riku). Some fights force you into that character for a short period. This isn't a big deal, however on critical, your allies are shit and just like in the main game, you're fighting 2 at once. The main issue where this is really present is the following fight: Aqua + Ventus vs Terranort + Vanitas.
Unlike in the other fights, you actually need to deplete one of their HP by about 50% before you can take control of Sora. The devs thought it was a very smart idea to only give 1 Potion, 1 Ether to Aqua. If you die in the fight, you cannot add more items. You're stuck with a very limited character. If you manage to get Vanitas or Terranort 1 v 1, it's not terrible. However, it's 100% always going to be Vanitas and Terranort against you, attacking at the same time, and one of the interrupting your combos on the other (mainly Terranort with his sliding attack). When I did my Lv. 1, this was the hardest fight. The part with Aqua is the hardest in Re: Mind, in my opinion, but only until you get access to Sora.
One very big problem fight is Armored Xehanort, where you're Sora and Kairi. Mechanically, this isn't a bad fight. What makes this a terrible fight is the camera. Xehanort constantly summons fire pillars beneath you. And multiple in succession. With the way this fight plays out, you're mainly in the air. When you're in the air, you cannot see the X on the ground when/where he's doing the pillar. They come out even if he's staggered. So you get ass blasted constantly. Later on in the fight, he starts summoning them from above you, where the camera doesn't appear. It's not a good fight, and I expect the data fight to be the same.