Personally, as someone in C and C++ for the last few years, memory access is almost never the root bug. It's almost always logic errors. Not accounting for all paths, not handling edge cases, not being able to handle certain combinations of user or file input, etc.

Occasionally an out-of-bounds access pops up, but they're generally so blindingly obvious and easy to fix that it's never been the slow part of bug fixing.

"Majority" could mean a few things; I wouldn't be surprised if the majority of discovered memory bugs are spatial, but I'd expect the majority of widely exploited memory bugs to be temporal (or pseudo-temporal, like type confusions).

I think UAFs are more common in mature software

There is use after free