I guess by libre office you’re not referring to Collabora?
Collabora is the online version of Libre, and absolutely integrates with mobile, nextcloud, web browsers, collaboration, etc.
I initially used OnlyOffice with my Nextcloud instance, but switched to Collabora with my own CODE server once I realized it was MUCH more thoroughly integrated into nextcloud.
Both are perfectly serviceable, but for the self-hosted storage/office suite combo, Collabora simply fits into Nextcloud better. Which is likely why you don’t see OnlyOffice discussed much.
Collabora is just more integrated. The NC and Collabora developers actually directly collaborate on integrating it into NC as the “official” office suite.
And AFAIK the backend of Collabora is simply LibreOffice, meaning the “desktop” version is: LibreOffice. The UI is the same, too, though they might’ve diverged since I last used LibreOffice on desktop.
Personally I’m not really concerned with formats, as long as I can finish documents as PDFs, and Collabora has brought a google-drive-like experience to my nextcloud instance that OnlyOffice didn’t manage. Either way I was able to do a google takeout of my drive storage, and just plop that into my nextcloud. But with Collabora, actually interacting with the resulting files within the nextcloud UI has been nicer.