Just purchased and set up a Dune Max 4K
the new GUI is a huge improvement over the stock GUI with the latest stable firmware.
so far this seems fairly stable, but have not used it enough to know for certain.
One nice change with this alpha firmware is that for Dolby Vision movies, it no longer continues to have to resynch; once DV kicks in, it remains that way. Also noticed I can now reverse while watching my 4K backups and full menus seem pretty stable as well.
One thing I wish the new GUI did was allow me to click on "ALL" and show all movies vs having to scroll a single list; I can get used to this new way, but I've always had a basic way of organizing my content. I can see the merits of this new interface.
the other thing is of course TV Shows. I do full backups and (when I use JRiver), I set particles to reference each individual play list (mpls) for each episode in a season. Here, I either have to remux into MKVs and tag the episodes that way or just play the entire disc and hope to find the right episode to watch.
Dune is forcing me to re-think how I organize my content, but the capabilities are pretty darn spectacular - 3D, Dolby Vision, Full DISC menu support. Audio I will continue to feed to my USB DAC from PC/JRiver, but liking Dune so far.