I've played 5 kick-off matches in Legendary difficulty (4 in Competitive Mode, 1 not), and the gameplay is certainly different. The dribbling is much more reflective of a player's abilities and way more consistent (no random touches, no weird animations, etc.). The contextual dribbling has added more variety and made it less monotonous. The more-consistent dribbling also helps with handling the pressure against the opposition defences.
The passing is also less wacky; the number of flicks, backheel passes, etc., have been reduced, and the animations aren't as weird as they were in 20. However, the passing itself has gotten quicker. I've played a bunch of passes that have zipped to the receiver despite me not necessarily inputting that much power or playing a driven pass. Through passes have also gotten more consistent (good), which has made them easier to play (bad). However, if you position your defenders well enough, you should be able to defend through passes. Despite them being easier to play, the Legendary AI are mostly good at closing down through passes, so I did have to work harder to break them down.
The shooting animations aren't as buggy as they were in 20, either, and taking shots under pressure isn't as easy as it was in 20. The AI defending has also gotten more challenging, which is a good thing as scoring goals was a bit too easy on FIFA 20 in general. They close me down way quicker whenever I'm setting up for a shot, and they don't tend to ball-watch as much as they used to. Having said that, I did win one kick-off match 7-1 against Newcastle whilst using Man. Utd.
Final note: despite Competitive Mode being branded as a more challenging version of Legendary/Ultimate difficulty, the game actually gets easier. Every team plays the same way regardless of the team, and every player does the same thing in their respective roles. The game's more fun to play without that mode (and more challenging, IMO, given the varied gameplay).