If building a custom commercial game engine these days... A team is 100% focused on the wrong problem, as the game-play content is what sells. Customers only care about game-engines when broken or cheating.

Godot, Unreal, CryEngine, and even Unity... all solve edge-cases most don't even know they will encounter. Trying something custom usually means teams simply run out of resources before a game ships, and is unlikely stable on most platforms/ports. =3

Many of those "edge cases" lurk in the platform abstraction layer (driver or OS bugs which needs to be worked around), and many of those problems are also taken care of in cross-platform wrappers like SDL (and for 2D games this is completely sufficient, you don't need UE5, Unity or Godot to render a couple thousand sprites and play music and audio effects).

But even more complex custom/inhouse engines are usually not written from scratch, those are often mostly glued together from specialized middleware libraries which solve the tricky problems (e.g. physics engines like Jolt, vegetation rendering like SpeedTree, audio engines and authoring tools like FMOD or WWise, LOD solutions like Simplygon, etc etc...)

>Customers only care about game-engines when broken or cheating

Most game engines are broken by default. Modern customers just aren't very discerning ("It's for the pigs. Pigs eat slop."). You can feel holes and rough edges in the vast majority of new releases, including AAA titles.

Unreal is the worst for this and Unreal-based games almost always have two things in common: a very particular, soft, sticky and unresponsive look & feel (often alleviated but never fully corrected by turning off some combination of motion blur, AA and VSync), as well as a UI that mishandles mouse pointers.

Unity devs seem to rely on a (more diverse but still quite) small pool of subsystems and renderers; possibly some mix of baseline and Asset Store components. This gives each Unity game a specific subset of flaws from a wider common pool. That is, you can tell that game A uses the same movement subsystem as games B and C (but not D), that game B uses the same UI subsystem as games C and D (but not A), and that game D uses the same rendering subsystem as games A and B (but not C).