I recently tried again a comeback to Ultima VII so I downloaded my copy and tried it in Exult, and the result is fine.

So I had a look at other engines for other games in the series and seen a lot got merged into ScummVM (UIV, UVI and UVIII with xu4 nuvie and pentagram) but I don't see what is the status

Are the games (and the spinoffs of UVi) playable and stable?

There is the eventuality of a gog re-release using those engines? (like kyrandia games that recently have enjoed)

Are there, in case, some up to date guides to get those games running on scummVM?

As long as you stick to unpatched vanilla versions, the scummvm "editions" should work alright. There are a few (not-too-severe) bugs but they are being adressed and worked on. Patched versions such as translated or otherwise recompiled binaries are liekly to cause trouble though.

Official statu: testing pending -> https://wiki.scummvm.org/index.php/Ultima

All open ScummVM Bugs for Ultima games: https://bugs.scummvm.org/query?status=new&status=pending&game=Ultima+IV%3A+Quest+of+the+Avatar&or&status=new&status=pending&game=Ultima+VI%3A+The+False+Prophet&or&status=new&status=pending&game=Ultima+VIII%3A+Pagan&group=component&col=id&col=summary&col=status&col=type&col=priority&col=time&col=changetime&col=game&col=reporter&col=keywords&report=9&order=priority