Under the hood it’s also way more similar: LibreOffice uses the odt format internally (and converts docx to it on the fly, sometimes leading to some formatting weirdness). OnlyOffice on the other hand uses docx internally, meaning better compatibility with MS Office (but also some weirdness with odt files).
(I’ve used docx and odt as placeholders for all the other formats here, like pptx, xlsx, etc.)