While LibreOffice has improved immensely over the years, its compatibility to Microsoft’s file formats is still a bit hit n miss at times, while OnlyOffice is a drop in replacement. It looks like MS Office and handles docx & Co. as well as MS Office, which might be a deciding factor for someone who has been on Windows for a long time and has all their documents in docx.
Also, I personally always get MS Office 2003 flashbacks when using LibreOffice, while file types don’t really matter to me.