For the time being, I'll keep using Dropbox instead of iCloud for syncing my files between my Macs. Here's why:
Storage in the cloud, and maybe on your drive
Here’s what happened: I was editing a podcast in Apple’s Logic Pro X, and my project was stored on the Desktop. All of a sudden, the voice of one of my podcast panelists simply vanished from the mix. I quit and re-launched Logic, only to be told that the file in question was missing. Sure enough, a visit to Finder revealed that Sierra had “optimized” my storage and removed that file from my local drive. I’ll grant you, the file was a couple of weeks old, and very large as most audio files are. But I was also actively using it within a Logic project. Apparently that didn’t count for anything?
So that’s bad. That’s enough for me to turn off that feature and never use it again—or at the very least, never keep my project files on the Desktop or in the Documents folder.
[Edited at 2016-09-22 14:19 GMT]