Even though I always ride with it, my UT800 annoys me plenty.
It fiddles the light brightness way too often, especially when riding in groups where others have lights. The battery life is short. It takes forever to charge and it needs to be charged after every ride.
I hope they improve the design on the next iteration. The smart light and control via ANT+ concept is awesome. Daylight visibility mode is fantastic.
Regarding the RTL515 - best piece of kit I own. The greatest step forward in rider safety this decade.