ONLYOFFICE is Russian company which, after Russian invasion to Ukraine, has created some shell companies to not look like Russian anymore. But still development and work is happening in Russia under name R7 Office (you can see both logos on left side). By the way, R7 is also name of Russian ballistic missile. If you use ONLYOFFICE — it is your choice — but please understand, you are helping Russia’s genocidal war against Ukraine.
Except for moral and political questions, there is also danger about data sovereignty and even theft if you run software from Russian company, especially if it used for important or sensitive data. Russia is like capital of ransomware, and R7 can give security holes to ransomware gangs. This is also why Russian developers are not allowed to maintain Linux kernel subsystems anymore.
ONLYOFFICE is also not real full open source, which increases risk for organizations. There are licensing problems (check isitreallyfoss.com). Some features like editing on mobile are only in paid proprietary version. If you look on GitHub, you will see almost all development done by employees, no active contributor community. This means if something happens with company, project maybe cannot survive.
Find some more proof in zip file here:
Other people already wrote about this — Ukranian company DMS solutions, also article in Ukrainska Pravda (you can find English summary). There is also topic on privacyguides.net with links to chamber of commerce and other evidence.
Quick visual comparison of the "two products" side by side, with left R7 Office and right ONLYOFFICE:
spot the difference in Text documents
Spot the differece in Spreadsheets
Spot the difference in Presentations
They are easy to compare... You can look in their documentation or other places, or try them out.