From my experience, not that frequent.
Had the occasion to watch a conference from a Dev in my company that would get rid of a lot of the overhead added by the Mono Behaviour in Unity. Getting better performances at the cost of less usability. In that case, one wonder why event bother using Unity?
Re:Qt Creator 4.6 Beta Released
by BoorishBears on Thursday February 08, 2018 @12:20PM
(#16333736)
For normal game logic I doubt the average user will ever cross that barrier.
C# in Unity can be blistering fast despite the GC if written correctly, and going to C++ still requires about as much (if not more) effort and knowledge as writing C# in a GC friendly way
EDIT question to Unity devs more experienced than I am: how frequent is it to "escape hatch" from C# to C++ game logic, in your experience? In what kind of game?