The company that makes them touts that you will NOT be able to break them no matter how much you tighten.
Whether or not you break your foot or ankle by cranking the hell out of them is another story, though. Heh, circulation will definitely be cut off before you break them.
I love my BOA system on my K2 Raiders and had this fear when I first bought them.
But after wearing them quite a few times, I have no reservations about cranking them down to the max. One thing I do actually worry a little about is resting my board's edge on top of my boot for support while on a ski lift. I have heard the weight and sharpness of the board and edge could possibly weaken or cut the BOA wire.