If CR123A cells are significantly mismatched, especially under higher loads, they can "vent with flame" which is not so good. If you use high quality, name brand cells, install them either into a single-cell device or brand-new from the same package into the same device, they're pretty safe.

I use a pulse-load battery tester to confirm that cells are matched. Voltage-based battery testers are not worthwhile.

If the user of a flashlight isn't willing to pay close attention to these issues, they should either use single-cell CR123A or AA/AAA lights.