Is a cohesive UI basically a dictatorial effort that needs a command-and-control effort from a powerful visionary force to implement? If so, that's not a good match for decentralized open source development.

Or at the least, more effort would need to be put in on the meta layer - how to enable a cohesive UI vision in a decentralized manner.

I'd imagine this is a different challenge than backend coding styles. There is still room for differing backend coding styles as long as certain principles are followed like following APIs, writing tests, etc. A cohesive UI requirement sounds much more stringent.