However, is it easy to work with an undocumented IDtech engine? Even a previous generation, say Rage? I think it’s very hard.
How many UE5 games run even remotely well? I've not played one.
Doom runs like butter on the switch.
Might be hard to run, I don't know, but at least it was well made.
The entire 2000s FPS scene was built on variations on the Quake Engine. And it got us Half-Life.