The problems I've had that come to mind seem to arise because I use iCloud Drive from multiple devices regularly: two macs and an iPhone.
I've encountered a problem once where a file version conflict could not be resolved; I had to rename one copy of the file to get it sorted out. I think it happened because I modified the file on the iPhone. Then, thinking the file had been already updated on the Mac, I edited there. Unfortunately, the update to the Mac took too long, so that introduced a conflict. The Mac was completely opaque about the problem; it just refused to sync, without an explanation.
Often iCloud refreshes are slow-ish. Sometimes, when I save a file onto my Mac's Desktop from my iPhone, the file will not appear on there, on the Mac, without my intervention, no matter how long I wait. The way I trigger the refresh is by creating a new empty folder on the desktop: the missing file then immediately shows up.
Even now, on my iPhone's Files app, next to the iCloud Drive main entry, there's an exclamation point with a sync circle around it. That indicates some kind of failure, but I can't find any way to learn what the failure is. It really doesn't inspire confidence. Other people have reported this and suggested various fixes, which I've yet to pursue.
There are lots of variables and iCloud is kind of fragile. If only 1% of iCloud's users have problems, then almost every user will have none (lucky you and them). But 1% of all iCloud users is an enormous number of people.