Well you have to buy a mixer that goes behind the wall which will add expense unless you get a faucet with a bridge in front of the wall.

The wall will have to be opened up to mount the rough-in, then fixed.

Wont be able to do plumbing in the wall if it is an exterior wall in cold climates.

I will be doing a wall mount, I figured "heck, its like a shower install, how difficult is that?"

Any other expenses? I don't know.