I'm a FIFA noob so feel free to take this with a grain of salt but...
- MM1-Karlsruher on WC. Waaaay out of my depth. Got killed regularly.
- MM2-Athletico Madrid on WC. Better, but still fired at end of 1st season.
- MM3-Barcelona on Professional. Spent the 1st half of the season around the middle of the table. Closed out winning the last 15-20 games. Messi led the league in goals, Neymar led in assists. Regularly scored 6+ goals. I picked up a lot of skills using a superior team on a lower difficulty.
- MM4-Wycombe Wanderers on Professional. Currently 2-1 and the games have been fantastic. I really have to earn my goals now(I've scored 1,2, and 0 goals so far) and I seem to be at a personnel disadvantage in terms of both speed & skill. The one game I lost was a 1-0 loss where my goalie inexplicably punched a ball at the post that would have gone out for a GK. Instead his punch knocked it back into the field of play where it was put in.
Point is...I think you have to strike a balance between difficulty, sliders, and the team used to get the best experience. I'm going to move my Barcelona MM to WC for season 2 while staying on Professional for Wycombe for now. The game doesn't perfectly suited for every user right out of the box nor is perfection defined by just a few difficulty levels.
You say that if you lose you want it to be because you didn't move your guy fast enough of something. Well, guess how often gamers truly believe they didn't make the play? Somewhere around never. The only way to achieve the sense of unpredictability is to randomly (occasionally?) give teams/players a boost/drop in ability. As all games do, it takes a little imagination on your part to concede that your guys are 'playing well right now' or are 'struggling' right now.