There needs to be a blacklist of items: Eureka effect, caber, spycicle, YER, deadringer, cloak and dagger, cow mangler, bison, pomson, and there's most likely a few more but i cant think of them right now too sleepy.
Eureka Effect: Allow red engies to get setup in their cells before the round starts
Caber: Can Nearly instant kill the light classes
Spycicle/Yer: The silent killing sorta makes it easier for a red spy to get a quick kill, and no one on blue will notice until it too late. I dont have faith that they will actually hit tab to see who is up, or notice the number players decreasing on the counter of the top of the screen.
deadringer: Can allow a red spy to break a rule quickly deadring leave the area and then wreck havoc, there's most likely another issue but again to sleepy to come up with anything.
Cloak and Dagger: Spy can just hide in the corner all day everyday
Cow Mangler/Bison/Pomson: Defeats the point of red trying to get into the armory, or find ammo boxes since those of unlimited ammo
Odds are these are some crappy reason but this is something I just now thought up and think might be some help.