It depends a bit on the engine too. A long time ago I designed & built a multi-platform 3D game engine that used quaternions internally for many things, but exposed Euler PYR angles for most "game-level" object controls. We (the engine team) asked around and ended up deciding that the minuscule performance gain and better mathematical behavior from using quats everywhere would be more than offset by some of the devs being unable to reason about how the enemy ended up pointing the wrong direction, when looking at values in the debugger.