The overall UI design has been consistent, what has rubbed me the wrong way is all the unnecessary UI features like soundboards, custom animated gifts/stamps, etc.

As far as UI hierarchy goes, its mostly been a challenge of the use cases for open source not fitting well with Discord. Too many channels, permission levels, and an unmanageable sidebar structure. That may just be an issue with scale of a larger OSS project, that's my only experience with it.