Hopefully, this helps someone out before they get into a situation like I was and rely on iCloud for syncing an important project. According to Apple support (and my own experience) iCloud Drive will not reliably sync project files for Apple's own professional software. I've confirmed this for Logic X and Apple support said it also includes Final Cut Pro X.
In my testing, some Logic X projects will initially sync or, randomly, will stay current, but they intermittently will stop syncing with no warning. Apple's only official response is that they are not supported.
In my testing, some Logic X projects will initially sync or, randomly, will stay current, but they intermittently will stop syncing with no warning. Apple's only official response is that they are not supported.