Retired LEO Officer (25+ years) here that was assigned to Highway Patrol DWI enforcement and was also EMT certified for that duration.
The mag charger or stinger were our general use flashlight tools. My use with them in an EMT role (think traffic collisions) was to asses pupillary reaction to light in cases of trauma or consumption of drug intoxicants.
For the DWI enforcement role and detection of Horizontal Gaze Nystagmus (HGN), I used a AAAA Streamlight Stylus pen light with a red light that fit in my shirt pocket. I had an attorney argue (in front of a jury) that the white lights were indistinct and would get lost in the crowd of the other lights around the roadway. The red light, when shown to the jury at trial, made a visual impression on them.
While there are several types of nystagmus, I only other checked vertical gaze nystagmus and lack of convergence with the red stylus.
While I didn't have an X21R, I did have access to a Nightsun when I needed it