Immediate Google results shows stun guns to be about 90% effective,

Google is 100% wrong. A Taser--not a stun gun--shoots barbed darts that are connected to the handset by thin wires. When you have good contact--that is, they aren't stuck in clothing instead of piercing the skin, and they're far enough apart--they're going to create strong muscle spasms, like a whole-body TENS unit. (Which, BTW, isn't that painful either, IMO, but you do lose a degree of voluntary movement. ) A stun gun works only when it's on contact with bare skin, and only works through pain-compliance. E.g., it "hurts", and the idea is that a person will want the pain to stop. Except that they don't really hurt.

This is a Taser.

This is a stun gun.

Ask any person that actually does serious self-defense training for high-risk situations, and they're going to say the same thing; you can not rely on a stun gun. A Taser will work, but you have exactly two shots, and getting both darts to make solid contact can be very iffy. Oh, and they only work as long as you keep your finger on the trigger; as soon as you let it go, the assailant is back in business. Tasers work in law enforcement because they usually work in teams and groups rather than singly.