Its (unfortunately) missing 90% of the functionality of Excel. It’s somewhat like saying “I don’t use Java, but what’s wrong with JavaScript”.
One of these days either Excel for Windows will work with Wine or they’ll get it working in a browser. Then I’ll go to Ubuntu and be happy with it.
What functionality do you use that’s missing from the Office 365 web app? I didn’t use it for a few years, then had to again recently, and was surprised how well it worked (in Firefox even!)
Namely, Power Query and some of the formula stuff (the entirety of the formula ribbon is just missing). I guess I could just start using pandas more, but its super handy to just click Excel, tell it where the CSV file is and start messing with something.