I think finals are the most difficult matches in general.
Because firstly, in one single game there is always something that can go wrong. Let it be your fault or Nordeus making the opponent GK a superman. If it is at this game, you are screwed... In other rounds you have a second game to turn things around and in league you have also always 2 games vs the same opponent, and a lot of games in general.
And secondly, I always noticed weaker opponent getting a boost especially for finals and suddenly performing much better than in other matches.