Lots of noise but nothing really concerning overall.
Related/relevant articles:
Ooph, OOXML are the XLSL, DOCX etc formats. First time I’m hearing that but good to know!
Love the fact that it has “open” in the acronym… 
But hey, most developer tools support those proprietary formats somewhat nicely. It’s a pain to deal with and Microsoft should follow standards better if we’re honest but that’s nothing new, been the same situation for the past 20 years or so?
Let’s be grateful for the people that maintain(de) those libraries that make it a breeze to work with those formats tho, definitely not an easy task. 
It’s totally understandable for software suits to use MS formats as a default, but it’s also true that support for ODF outside of LO is quite poor. And OnlyOffice can be worse than some other (proprietary) programs.
It seems to me that OnlyOffice quickly became most popular MSO alternative, and I would say for good reasons. It’s open source, available for cloud integrations, has simple interface, similar to MSO, and although not so feature-rich, has the most important ones, some that even LO is missing.
I’m still sticking to LibreOffice, but I understand people and organizations who prefer OnlyOffice, it’s hard not to recommend it.