All of these engines (Unreal, Unity, etc.) are written as monolithic "frameworks" that want to own the top loop and have you override certain bits to customize

All of these engines (Unreal, Unity, etc.) are written as monolithic "frameworks" that want to own the top loop and have you override certain bits to customize behavior. Also, the nature of games requires that various components (graphics, physics, ai, networking, etc.) are tightly coupled together. So, no, you're in a world of pain if you want to use Unreal and not do things "the Unreal way". --- Honestly though, I see nothing here that couldn't be done in any modern engine out of the box though. They can all stream assets in and out of the level asynchronously (for "infinite extent" levels), build meshes/textures programmatically, support fully dynamic lights, etc. If you have some requirement for procedural generation that isn't supported out of the box (e.g. baking lightmaps at runtime), surely it'd be easier to just write that part yourself and inject it into the existing pipeline.