There seems to be widespread disdain for it. AFAIK they switched to a shared codebase to make it easy for the devs to deploy to multiple devices. But, it came at the expense of UX for pretty much all devices. They don’t use native video services on things like iPhone.
I use Pleezy which has been a nice open source alternative to the Plex UI. The only issue we've had is that on wake, you have to press back once to do anything else. I'll eventually debug it and see if I can fix it.
The new UI version never made it to the Xbox client so that's what I switched to using about 6 months ago. As a bonus, I got to ditch my Roku altogether.