Yeah, I switched to the web version of outlook when I noticed (via little snitch) that the desktop app was sending 100's of MB upstream. No idea why it'd need to send much of anything in that direction.
I think I was mainly upset that it was using a bunch of upstream bandwidth (if I remember right, it kept going while idle), possibly also didn't like the disk space that it was using for caching. Nothing important. Arguably a web application is more secure, since it can't read my disk, but that wasn't my concern.
At this point, I prefer the UI of the web app (I tried switching back), and since it is a PWA, it behaves like a normal application.