I am more comfortable with JS + Three + Web than Unity/Unreal, and we had planned this as a web first experience. I also wanted a lot more control over the application than what those engines support (eg - in terms of loading, mobile, asset management, DOM-based UI, etc).
In hindsight some things would have been a lot easier with a game engine!
My question is, what factors made you decide to hand-craft it instead of using Unity/Unreal?