1. No way to re-sync existing folders on your computer (seriously). I was pushing hundreds of GBs of photos to my Google Drive, and after syncing everything, upgraded to a new computer (the files were stored on an external drive locally). You'd think you should be able to reconnect to your account and point it at the files that already exist locally. Nope. There's no way to do it - the client tries to be so "smart" that it will refuse to recognize existing files, and tries to re-sync both copies (both ways) as duplicates.
2. The desktop client consumes nearly 100% CPU while syncing. I realize that while scanning local folders, the client may need to run checksums on files (potentially CPU intensive), but Google Drive was consistently eating up extraordinary CPU resources even after scanning the local files. I would expect some occasional disk IO and high network IO, but not constantly high CPU usage... who knows what they're doing.
What bothers me most is that Google appears to be completely ignoring these issues, even with hundreds of posts on their forums over years with people complaining about these things.