I've done this numerous times with those external boxes and it works fine. As long as the box is deep enough it will work fine.
I never understood why they dont come up with a clamp that is made for this application, that dont require such a large hole in the wall.
I always put a bead of silicone caulk around all external boxes after it's mounted. That will keep water out of the box, because otherwise it can follow the romex into the box regardless how the clamp is installed.