I don't really know in Qud's case, but sometimes an engine helps with some surprisingly basic stuff.

I just went from hand rolling my own engines to using Godot, and it's just incredible how nice it is to have something move my textures in and out of the GPU for me instead of having to do my own texture atlassing and preloader. I could rewrite the entire renderer and game loop by hand and I'd still be gaining stuff like that.

Still probably easier for Qud to switch engines than it would be for a lot of games. A lot of Unity devs don't even program and are just stringing together a dozen no-coding packages they found on the unity app store.