See, the that's just the thing with turrets, they can turn on you. Their programming is so hard to predict, they might as well me with the zombies.
Worse really.
It's life playing Evil Genius, and putting a deadly trap in a high traffic area. It might never get an enemy agent, but it's going to get more than it's share or your minions. Just by virtue of always being there and being dangerous, it's more dangerous than occasional threats like zombies and S.E.A.L. teams with weapons of mass destruction. Stupid S.E.A.L.s, are always blowing up my armories, they're almost as bad as ninjas.
Black holes obey consistent laws of physics, and only spray fountains of deadly anti-matter at regular intervals, and only out of their poles. You can predict those, so naturally they are less dangerous.