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.

That is true, but it's safe to say that the number of people who do understand matrices is far greater than the number of people who understand quaternions. It's also worth pointing out that game engines and graphics APIs all deal in matrices, but don't all deal in quaternions. Mats can do what quats can do, but it's not true the other way around, quats cannot do everything matrices can do. Quats don't help with perspective, non-uniform scaling, or shear, just to name a few.