I think personal experience is everything - I can't play the game every day so I'm not a superstar, and though I used to find Professional too easy, I've realised that it's probably because I was playing as Liverpool (primarily because I half-support them, but also to see all the Premier League stadia in their glory and not get bored of seeing the same stadium every week).
I played some more games last night, and won against Everton 1-0, but comfortably, which concerned me. Then, back to the international games, as Ireland (against France), it was a different game (in a way that I really don't believe you get to see on World Class and Legendary) - they steamrollered me, and it felt right.
Then, back to Liverpool, and against West Ham, it was a bit too comfortable again - so I gave them a slight speed advantage, less first touch error, less pass error and less shot error, and although I still won, it involved a little more work.
It works for me - but it'll be different for everybody!
For the first time in a while, I cared enough to look at my remaining fixtures and figure out where I can finish. I'm fourth, behind Man Utd (two points ahead of me) with Tottenham and Man City behind me (with two points less). My next two games are against Tottenham... Then Man City...!
The last five games are against teams in the bottom half. I'm excited to see what happens (although I can't stop Chelsea, who are top by 10 points - guess EA didn't see that one coming)...
The game, as a whole, I absolutely love - I just wish that it was better tactically, defensively, and that the AI was more "I" and less "Burnley become Barcelona" (on the higher difficulties).