>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).

In my humble opinion, the difference between good and great was often whether the Shaders and pre-Baked work was done well enough to go unnoticed.

Forcing devs to use a mid-grade GPU also tends to reduce chasing performance issues later. For example, high frame-generation artifacts users often perceive as "floats" or "wobbly". =3