The thing about custom engines is that by the time it supports all the features you need, you’ve worked your mindset into optimal engine usage, so you end up being more performant on a hacked engine vs suboptimal code on commercial engine. Home grown always has some unique flavour.
It might be true in some very special cases, but if a programmer can't make games like Crab Champions (the example used in the grandparent comment) performant with Unity/UE, they should be nowhere near to a custom engine.
