The threshold can be a problem- you don't want the threshold to be too low, or else the brake lights come on when even slight deceleration is occurring. This would give an annoyingly too-common signal that would possibly be disregarded/ not be an effective form of visual communication.
As I understood it, it's less of a real problem and more of a perceived annoyance. People accelerate, let go of the pedal, realize that a deceleration sets in, accelerate again, etc. so their brake lights are flashing all the time (instead of just using their cruise control to have this happen much more smoothly and ideally below the relevant thresholds).
