Well my thought (remembrance) was that these "infinitely variable" UltraFire drivers had bad PWM. I just did a search and quickly confirmed PWM was bad on the UF-H3 (I assume the UF-H2 too), but "fast and unobtrusive" on the UF-T50. I own the UF-H3, and I've never noticed the PWM (I'm not easily susceptible to it), but I've never used it on a bike either (it's really not appropriate for bike use).
Perhaps (hopefully) I am wrong and the PWM is high enough not to be an issue.
-Garry