I was playing a terror mission in newyork and there was an unusualy high number of friendly npcs (around 8 civilians and 3 soldiers) and a single reaper managed to infect 8 of them causing complete havoc, andvi thought it would be interesting if every few terror missions there was one with a large number of civilians and the only enemy was reapers.... they are probbly the most dangerous oponent in the game but you never see them in large enough numbers to be a real threat