I am a certified instructor for the pepperball. I love the thing! It, like all the other less-lethal weapons with which I am familiar, is just another tool for the toolbox.
I'd have no concerns regarding the pepperball because you train with multiple rounds with each deployment. Heck, I do the same thing with my firearm. Nearly all of our handgun training is in 3 shot groups! Does that make my handgun less effective? Hardly. As anyone who has been hit with a pepperball will tell you (which I have), you start feeling the pain at about the time the 3-4 round is hitting you. It's very intense and very effective!
No, there is no garuntee that it will stop a determined subject or one that is high on drugs. The same can be said for handguns.
Yes, the rounds can (do) cause some serious welts and may break the skin. Yes, you can lose an eye. They should not, however, be lethal. There are do***ented incidents of deployments on a subjects head (temple area). The subject was knocked unconscious, with no permanent injuries. It was a lethal force situation and the officer made due. The guns are marketed by the company as NON-LETHAL. I do not like that term and tend to stick with the term "less-lethal" as with our batons, pepper, tasers, etc. Why tempt fate! 
BTW, this is not just a paintball gun on steroids! The accuracy was phenomenal. We were hitting soda cans at 30 feet with no problem. The pepper was surprisingly potent, even outside.
There are liquid rounds for pain compliance, pepper rounds that add pepper to the mix, dye rounds to mark individuals, as well as solid, nylon rounds to break glass!
It's another great tool for the toolbox!
We bring evil things to evil people, kicking in a door near you!
."In theory, there is no difference between theory and practice. But,
in practice, there is."
- Jan L.A. van de Snepscheut
"The difference between 'involvement' and 'commitment' is like
an eggs-and-ham breakfast: the chicken was 'involved' - the pig
was'committed'."
-unknown
Working on a PhD in CQB one doorway at a time.
When the wolf attacks, he will find not all who run with the flock are sheep!