

From what I can tell, OnlyOffice has the best compatibility and the nicest UI (similar to MS office), same as with the regular applications. NextCloud Office is based on LibreOffice (officially Collabora, which is their name for the web product), so again same as the regular applications you’ll have some compatibility issues. That said, if you don’t need compatibility with existing documents or only documents made with LibreOffice, either is fine.
One concern many have is that OnlyOffice is closed source (edit: my bad, it’s been open-source for a long time) and russian based (edit: partially russian, see Wikipedia), while LibreOffice is open source.
I was programming in assembly for ARM (some cortex chip) and I kid you not the C program we were integrating with required 255, with just 1 it read it as false