yeah I love when entire teams decide to play as the same class. this equals epic fail in most cases. The game is a class based game for a reason; to be a good team you need a good balance of all classes.
The assassins are easily predictable, I played assassin for a couple games I liked it but now i play assualt. I just predict there pat hand throw a bomb and dtonate which usually reveals them and I kill them. Yeah if a team plays with like 4 or 5 or maybe even 6 of the same class, just pick a strategy and class that counters them. Though support is hard to deal with if there's enough and they know what their doing. Just 1 or 2 support with a couple a bots is tough along with their turrets sitting in corners.