Aside from the standard dark patterns and extraneous, feature-forcing bloat, I really dislike that flat-design trend. Not just does it look kinda bad, but everything is so physically bloated to try to separate it from everything else that you get notably less usable screen space resulting in annoyingly little information density.
That’s definitely a part of it. All of the MS web platforms rely heavily on first and third party cookies. You can’t even log in properly to Teams if your browser is in incognito/private mode.