I've seen an NMO mount on a truck cab roof, get hit and stripped a slot right down the roof like a shear would do. Tough mounts, I just don't like roof mount. I've had issues before. Minivan roof mount hit garage door that didn't go all the way up, bent over into the roof - everything was loosey goosey after that. Added a large washer to the inside of the roof helped, but never could use the mount with anything other than a rubber duck.

I've got two of these, used on multiple vehicles, easy to mount or move to another vehicle. No holes. No leaks.

http://www.diamondantenna.net/k400photos.html