I won't use assisted online but do me a favour, play on semi controls, as Tranmere against Huddersfield (both in the same league) on world class, and tell me that you're not frustrated as fuck after five minutes at how they keep pushing you off the ball, pressing for 90 minutes without getting tired, and scoring goals that you can do nothing about (pinging the ball around you like Premier League players, you can't get near them). Plus, notice how your passes are underhit and intercepted all of the time, and if they're not going to be intercepted, the game will delay your reactions enough for the CPU to grab it.
And when I say "Tranmere v Huddersfield" make sure that's the game you play, because if you play Liverpool v Everton or whatever, it's a TOTALLY different game. They have footballers who can actually play football, Tranmere don't. But neither do the other League One teams, yet they play like heroes.
I only switched to fully assisted in desperation, not thinking it would do anything, and now I've gone from 0-1 0-1 0-1 0-0 0-1 (0W 1D 4L) to 2-0 0-3 1-2 2-1 0-0 (2W 1D 2L). A lot more realistic and a lot less frustrating.
Now if I played Liverpool v Everton with all-assisted it would be stupid, but playing in the lower leagues, I have to - otherwise every game is the same frustrating horrible game. I'm convinced it even changes the AI of the opposition a little bit.
(If you can get used to playing on all-manual though I'm sure that's a better game on all difficulty levels - I just can't get used to it though, I've tried and I'm not accurate enough to do anything)