I’ve been testing it and it seems like a good solution for general productivity and a great option for people migrating from MS. It’s open source and cross-platform, but I just don’t see it in any conversations about office software.
For me, it’s so far leagues beyond LibreOffice. I really need something that works on my phone and syncs across devices, and allows collaboration. OnlyOffice seems to fit the bill. It’s also far more intuitive to my preferences.
I am sure that some people wouldn’t like the fact that the interface runs as a webapp, or use of Java, but it’s strange to me that it’s not usually even in the conversation.
OnlyOffice is also available as a (offline) desktop office suite, so as a user it’s more like LibreOffice than Collabora.
My wife prefers OnlyOffice over LibreOffice, as it’s feels closer to MS Office. Also it’s compatibility with MS Office formats is better than LibreOffice.
I still use LibreOffice for desktop, but I’m not a heavy user. Haven’t tried out Collabora or hosted OnlyOffice for NextCloud yet.
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.
OnlyOffice is super easy to integrate with Nextcloud, and was their original office selection prior to the commercial opportunities with Collabora.
Also, OnlyOffice is built specifically for web rending, while Collabora is a pseudo VNC session. There are advantages and disadvantages to this, but they are very different systems under the hood.