It can't handle OpenDocument files (odt, odp, ods) well. Only a one-way import is supported. https://helpcenter.onlyoffice.com/docs/userguides/document_e...
There are parts that are not released under a Free Software license but that's unclear from their marketing-communication. #openWashing
https://www.softmaker.com/en/products/softmaker-office is a thing as well, and I prefer it to Only Office. The maker is German.
It's a bit slower performance wise
Any gotchas to be aware of with OnlyOffice?