It’s generally considered bad practice only for bash and similar commands that execute their input. It’s not a bad practice at all for commands that just display or transform their input, like `man`, `less`, `ffmpeg`, etc.
> ffmpeg There is certainly a few hundered exploitable vectors in that program alone... to say nothing of the rest.
When in doubt, spin up a VM to run the random untrusted thing -- And then go read its mailing list/issue tracker for known VM escaping exploits. I have a machine setup to test malware, so I just hit my "airgap" switch to isolate the system from my network once the questionable code is in place and ready to run (potentially amok). Study-up about ARP-poison attacks, and remember ARP does not transit to upstream routers/switches (Y "combinate" your network for fun and profit).
Before you assume non malicious simple text output, consider "ANSI" escape code complexity as an intrusion vector for whatever terminal you run. I've got "0-days" for this going back to MSDOS: ANSI Bomb => arbitrary CMD entry. You don't have to take my word for it, your terminal of choice is most certainly vulnerable to some ANSI/escape code related exploit, look it up.
Wait a minute I just realized there could be a zero day in the VM hypervisor too. I guess I'll just have to buy a fresh Raspberry Pi for each file I want to open.
/s
(Also, I don’t know about ffmpeg as it is somewhat more rare to have it be public-facing, but there have definitely been exploits against ImageMagick, usually targeted at websites using it to process user input.)