I have been using a MD locker for many years now and never had a problem. There are no electrics, just 2 air/vacuum lines, and they are pretty tough. I once had a huge branch wedged in between the rear axle and the locker lines. Didn't do any damage.
Only person I know who ever had a problem was someone who ran the lines too close to the exhaust when installing and managed to melt them (but that was installation error).
I know of a few people who have pulled auto lockers out because they are unhappy with the road manners. Also, if you break an axle, auto lockers often break at the same time.

