>reliability
Most of the time it will make someone freak out, but the reaction varies from "oh shit" to "I'm gonna beat your ass twice as hard".
There's an acceptance by cops that tasers, specifically the ones with wires and darts, will not work more than half the times you deploy it, and they use good shit. That's why they're smart and use a gun.
There's also a chance you're dealing with a lightweight and you end up killing them. Not as rare as people might think.
>negatives/drawbacks
If they're wearing a jacket you have to find a weakpoint.
If they're already on top of you, they probably won't be persuaded to get off by being zapped and just lay onto you harder.
If they know you have a taser, they're either going to wiggle around to make it more difficult or beat the shit out of you.