The UI is fairly simple, the engine is fairly complex, with a lot of interactions between different parts.

I don't think most games struggle because their UI is complex, or because user behavior is complex (there's really only so many things a player can do, especially general ones -- most variety of interactions are "scenes", entirely captured within themselves). At least, most AAA bugginess isn't usually found in their UI. And graphics aren't the complexity generally -- they're just visual artifacts of the engine itself failing.