A two-stage one:
A while ago I installed Monterey as a second boot option on my Mac Mini that came with Catalina, which has been how I use the machine since. However, that Safari version is falling behind noticeably and the new features in the upcoming version of the OS look good, so I pretty much decided to go with the flow and run an OS created this decade.
So in preparation I upgraded my Monterey install to Sonoma. And then a litany of iCloud complaints started: it wasn't syncing (I don't care, but apparently this is quite important so nag the user constantly!) and then also when I boot back into Catalina.
(Bonus: I need to re-pair my mouse after each OS switch. And holding down alt on my Drop CTRL keyboard or wireless keyboard doesn't let me choose the OS when booting, so I need my trusty Model M's alt key for that.)
Anyway, I have to enter a plethora of passwords and passcodes over and over again. That is stupid. But perhaps 13 times will be a charm. 14 then?
But the REALLY annoying part is the two factor stuff, where Apple randomly decides which other device I need to handle the second factor. Which is regularly my iPad. Which is not in my home office, where the Mac Mini resides, but on the couch.
Well, it used to be, now I can't find the damn thing at all. So no iCloud access for me.