The worst I think you can say is III was an over-correction for II, or Fish was kinda wasted. And it really comes down to directors. You end up with MI:II-Hardboiled, and MI:III-Alias.

The series doesn't really come into it's own until IV. V and Vi had the same director so of course they are more cohesive AND they built off what came before rather than did something new.

Taken as a whole, it has to be one of the most consistent and enjoyable movie series out there.