I've heard that the standard Lemmy UI is not under active development because the Lemmy UI developers are working on a rewrite. I looked at it for a while though, and I thought that, aside from some missing features and polish, it was fine.
I'm trying Photon right now, and it is also fine, and a little more feature-complete, but it is visibly janky in some respects. Maybe it is my biases, but I also much prefer the model of clicking on things and getting a new page over a single-page app that you interact with via controls.
What was so terrible about the vanilla Lemmy UI? Do people really have a strong level of dislike for it? It seems to me like it just needs some love to smooth down the rough edges and awkward spots, but the core doesn't seem in any way terrible such that it would need to be abandoned.
You can read our discussion and reasoning for this here.