In the ME universe they're powerful, but if you look at the power of their canons and the strength of their armor, it's no where near other Sci Fi ships. The indoctrination would help, but is might not work on the other kinds of beings mentioned here.

I giant fleet of Reapers soaring down to destroy a planet is pretty devastating stuff. You need some heavy duty weaponry to take one down that has it's shields up. Especially the Harbinger's.