They have the disadvantage the they can kill someone who has a weakness and have little effect on someone juiced up on some drug cocktail - well done.
Whatever you pick, it's going to let someone through and be too hard on someone else and there is no reliable way to tell, a priori, which is which.
eg. you need to be psychic.
So you relax the criteria - you want something at least as reliable as a sidearm or a shotgun but less likely to kill and less chance of bystanders being affected. This is a work in progress.
It's also why I asked the question: what is the problem +Sai
is thinking of, that the kind of weapon he's proposing is thought to solve?
The only thing I can think of would be the situation where you grab your taser and realize you should have grabbed your firearm with no time to swap. (Usual answer is to run away - or hope your partner is on the ball.)