Actually it's both instant UX and far simpler code. With Zero I define a table schema, relationships, and permissions in beautifully simple code, then I also write my queries including relations with a nice typed query-builder, and I get not just instant UX but also perfectly fetched and synced data with permissions and mutations that are fully consistent without any extra thought.

Doing that with any other system, sync-engine or not, requires a huge mess of code and ends up implementing some sort of ad-hoc glue code to make even a part of this work.

I'm building an app right now with it and I'm currently so much further ahead in development than I could be with any other setup, with no bugs or messy code.

Your choice is either 100% server-side like v1 Rails, or some sort of ad-hoc sync/update system. My argument is either you should stick 100% server side, or go all the way client properly with a good sync engine. It's the middle part that sucks, and while there's a chunk of apps that benefit from fully server, it's not really an argument that you can build much faster responding apps client-side and that users generally prefer it, rightly so.