Kingdom Hearts fan Everglow8444 has made a video examining the changes made for Kingdom Hearts HD 1.5 ReMIX. He finds that Square Enix has cut corners all throughout all three games.
The most glaring issue out the gate is that Square Enix is misrepresenting the quality of the ports with promotional screenshots that have been altered to look better than they actually do in-game. He also found that the graphics in the Kingdom Hearts Re: Chain of Memories cutscenes actually got worse. While close-ups were given the proper attention they deserved, but they get blurry from a distance or when moving fast.
Another common complaint he has involves the now widely known two facial animation styles in Kingdom Hearts. One is more detailed and expressive, where they react appropriately to the scenario, for the cutscenes. The other style is used in-game, where they have blank smiling expressions, even when the scene does not call for it. Everglow8444 thinks the latter looks even worse in HD, but Square Enix left it intact.
He also notices that Theater Mode has been removed for Kingdom Hearts Final Mix. While not necessary, this is a glaring omission for how easy it would have been to resolve. Even more damaging to the narrative, however, is the omission for battle scenes in 358/2 Days. This game is presented in this anthology as pure cinematic; however, the cutscenes are merely placed together in sequence without any indication where fights took place. As a result, the story is incomplete, and even looks disjointed.
There are even more odd omissions that singled out certain games. Again, 358/2 Days, as well as Re: Chain of Memories, suffers from having less tracks remade for full orchestra, whereas all the songs from Kingdom Hearts Final Mix got this treatment. Getting back to animations, they updated the models for Sora and Ansen in Final Mix, but failed to provide the same treatment to Re: Chain of Memories.
Everglow8444 makes some more observations about some odd gameplay quirks, but I think at this point you would have read enough. The evidence seems to be abundant that Square Enix made compromises for Kingdom Hearts HD 1.5 ReMIX. If you look back at their extended delays for the Final Fantasy X/X-2 HD collection and their recent struggles with Final Fantasy 14, Square just seems to be trying to do too many things all at once. I know hardcore fans will still want to get this regardless, but be forewarned, this Kingdom Hearts anthology will have some serious problems.
Source: Reddit