The biggest UX issue, in KDE in particular (though they made some improvements recently), is that a copy process will say it's complete (with a notification), but you could be waiting several seconds or minutes before it's actually done syncing

The sync command will block until it's done so you will know for sure. I believe ejecting the drive will sync before it reports success also.

In my experience, simply waiting for the activity light to stop flashing is a good enough indication, but not all drives have one.