Maybe I should have been a bit more specific, but I was mostly just talking about UI features and customizations. I have no problems with a ROM modifying stock

Maybe I should have been a bit more specific, but I was mostly just talking about UI features and customizations. I have no problems with a ROM modifying stock Android under the hood if it's necessary to enable basic hardware functionality. The point is to keep modifications to a bare minimum, to limit the overhead required to support each new Android release, and to maximize the number of devices the ROM is able to support given limited resources.