I had one
and speculating, since what the 'bashing' type eat are snails, shrimp, and small crustations. You'd have to take out anything good snail/shrimp/crab wise from the tank for the duration of the mantis' stay. My mantis was well fed, so let my one single snail live in my ten gallon tank for him.
The real minus to having a mantis in the tank are... well, a few.
no more hands in the tank-they are called thumb splitters, although the smaller ones won't do much damage-a nice bruise, not much breaking skin. (I don't know if this is an actual minus, as many of us need help keeping our hands out of tanks!)
no more snails, unless you find types with impermeable shells.
They tend to rearrange to their liking,not yours. That nice small colony of zoanthids MAY just be moved to the back of the tank. And remember, no hands in the tank.
Thats about it, really. nothing TOO bad, I liked mine. The smashing type also don't go after fish unless they are pretty much starved, mine lived happily with a green clown goby and a blue damsel. Their lifespan is decently short, just a few years.