Why is Pandemic's Bio-terrorist Challenge not recommended with five players?
Good question! Amazingly enough, there's a definitive answer. The reason is described in detail in this board game geek thread, where game designer Tom Lehmann explains that playtesting showed the game became too long and boring with 5 players. Basically the game becomes 50% longer, and there are large gaps where individual players don't get to do anything. The detailed analysis he provides is quite interesting.