Is it foolish for me to use iCloud Desktop to store personal documents? The only reason I do it is (a) to sync between desktop and laptop, (b) have access on iPad and iPhone, and (c) as a catastrophic-failsafe
I'd suggest using software from an actual software company that cares about the quality of the service. Sync.com, for example, does only one thing (not like Dropbox, which is rapidly adding features no one cares about), it's end-to-end encrypted, and it's based in Canada.