The problem with devices of this type is that (the ones I have seen) present a flat screen without any focus adjustments or exit optics. Which is to say, no "eye relief distance" and no "eyepiece" focus adjustment. Just a small display to stare at.
This leaves you to focus your aiming eye at a distance of several inches while zeroing in on a target much further away where your two eye focus belongs where gunplay is involved. "The target and what is beyond" takes a back seat to the screen..
It is like watching a TV screen that small and close to the eye.
If I were still working the street and wanted a pocketable termal for checking alleys, closets, etc. for hot spots, I'd always have one along.
However, unless one of these devices comes along with both the thermal image and the aiming reticle projected to a large distance, I wouldn't want it on a firearm.