Yep, it is "luck of the draw", and the default hero brain, while very good at gathering diamonds, does tend to be vulnerable to the "assassin" roles.

Eventually we'd like to add a slightly smarter matchmaking system (assign users to games based on some sort of performance ranking perhaps?), but for now it's very possible to end up on a less-than-stellar team.

What's so strange about that? The majority are probably bad players.