Data collection. I can see MS collecting all kinds of "metrics" to "improve user experience". That is assuming all processing happens in the phone and not in the cloud in the first place, which the article does not mention.

You already have to trust MS implicitly if you're using Excel though. And imagine how many .xlsx files are flowing through OneDrive already, filled with passwords, PII, SSNs, etc.

That is true, but at least one can use Excel offline or isolate the process from networking for a decent level of security.