If you work for a place where security matters, your default assumption should be, "it's unsafe".
If you work at a place where security matters, then mounting external volumes is disabled in the group policy along with auto run, installing software, and 99% of Windows tasks that aren't directly related to your job.
As he said, he blames the people who designed a system where plugging in a usb stick is unsafe, since the logical thing to do with media is to see what's on it. But for now that problem exists and people should be exceptionally cautious about such things.