snowee I only use Onlyoffice when the file I work with just doesn't play well with Libreoffice. It works best with the microsoft document that has weird formatting (especially MS Word). Honestly, I don't really like the interface. Sometimes it feels slugish to navigate around a document. Maybe I got used to Libreoffice, but that's my experience withe Onlyoffice (Which I don't have installed right now because I have not encountered troublesome file in sometime).
Also about why it was not in the repo. We have a package request for it here, but their licensing scheme is annoying. (Disclaimer: I am not a lawyer or anyone practicing law, so my interpretation might be wrong). Sure it is AGPL 3.0, but they include exception that forbid anyone that redistribute it to use their trademark. So, in a sense if we want to include it in our repo (redistribute), we have to strip down all of their trademark from the software (logo, name, etc). So, the chance it is included in Solus repo in slim to none.
Now, you might ask: Why it is in Flathub? The answer: they just repackage the official prebuilt binary, which we can do(technically). But then it will not have any benefit of being a native package. So it is what it is.
