With a seatstay mount, you will be adjusting it, as the light will be bumped and moved around the stay. If you're the type to check things often, that could be OK.

Consider using more than one tail light. I always use an under-saddle tool bag, so I attach my blinky light there. I also use a spoke light, and it doesn't have to be bright. The motion of the light is a very good attention getter.