Google Drive does seem especially flaky in the way it complains about "unsyncable" files and doesn't bother to even give me an error code to investigate.

They aren't "unsyncable" per se, but Google Drive has preferences. For example, I could not sync files from a stackable encrypted filesystem (EncFS or eCryptfs). Files that go just fine through Dropbox or SugarSync. Same "error". I guess there's no money for Google in files you can't take a peek at.

I feel Picasa is in a semiabandoned state.

I quit Picasa when they dropped support for Linux. Granted it was running under wine, but it worked.

I thought so as well, but they just pushed out an update at the end of March. https://support.google.com/picasa/answer/93773?hl=en