Compared to single player (offline) which can be a joke when the AI decides to play far in excess of what you'd expect.. on co-op you get situations of just straight up cheating. This is on World Class difficulty (with standard sliders).
Rubber-banding is very prevalent when attacking.
The human controlled player making a run off-the ball is often dragged away from where you want to run, to towards a defender to help the AI defence keep it's shape, only once you've run offside do you get free control again, however as soon as you've run back onside the AI defence are in that players face immediately (even if not in control of the ball). In this circumstance (when on the attack but without the ball) the AI forcibly slow you down either by the defender seemingly grabbing your shirt or by some form of magic (maybe a random patch of mud has appeared in-front of the player) any space you attempt to run into (where no defender is covering or near to) will see the human controlled player slowed by one method or another allowing the defence to organise.
Marking (as partially mentioned above) is outrageously good from the AI.
I'm not talking about playing against Chiellini or Varane here... i'm talking about defences who have mid/high 60 rated defenders at best yet stick like glue (as if they all had 90+ tactical awareness) when you go forward, now to be impartial and fair this doesn't happen all the time however this can be offset by the fact that when they don't get incredibly close as soon as the ball is passed towards you the AI go into super-speed mode where by they will stop you running past them in any situation and show what can only be described as super-human strength. Playing as Wolves v Stevenage (season 1 FA Cup 3rd round) and Adama Traore (playing as a right winger) could not out-sprint their left back, we playing numerous high/low through balls that saw him put in, yet time and time again he would be caught up and knocked off the ball and that's just one extreme circumstance that was ridiculous.
'Gliding' or 'Skating' rears it's head all too frequently too, players making up 2 steps worth of distance in 1 animation, this is also an issue when your team is defending counter attacks. One of us running against the AI player with the ball, the other marking a striker in the center yet the striker will almost always be able to pull away even when their player is physically more tired and a worse player (stats wise) than the human controlled defender.
As much as it makes for a harder (sometimes fun) experience it becomes so frustrating at times you just can't be bothered. Would it be as tough playing as someone like PSG or City? Maybe not but i'm sure all the above points would still rear their head. This isn't a sole FIFA 20 issue either, on 16 & 17 (didn't try co-op on 18 or 19) it was a problem too. The above situations can happen in reality i'm not arguing it's not realistic but not when they happen game after game, I've witnessed the above happening sometimes 4 out of 5 games, the rare game you get that seems like it's a fair challenge gives you faith but that's all too short lived.
It stinks of the AI being programmed to limit what the human player (off the ball) is capable of doing to allow itself (the AI) to play as if in standard single player (controlling a team) mode.
For those reading this thinking it can't be as bad as i'm making out and it's likely we're just bad FIFA players (and that maybe so) i encourage you to try yourself's.