Probably not so much in aircraft (although oil collection in the cylinders would likely be as much a problem in inverted V-12s as in radials), but it would be a noticeable pain in ground vehicle and marine installations. On the other hand, conversion forright-side upupright use would probably be fairly straightforward, albeit not trivial.
Same problem would be in boats.
Have ever been a V-engine converted into an inverted V or vice versa?