HoSac is simple, it caps out at 100% your max health OR lasts 12 seconds.
Yes, it can easily kill you if you're not aware of your own health. Nef Crackle, Chimaeron pre-Mortality, Atramedes (when you have high sound before Modulation), certain add mixes of Halfus, Cho'gall (p2), Council (p2). Basically use common sense, since it can kill you. There is nothing baked into the skill that will prevent you from dying to the damage taken. In most cases PotI can easily compensate for the damage taken unless it's very large unavoidable raid wide damage.
It works beautifully in most cases especially on a tank who mitigates/avoids a lot of damage taken. It works great to alleviate Mortal Strike debuffs on tanks. It will break Polymorph, Sap, Blind, Repentance, Seduce. It won't immediately break Fear, Hex, but may help to reduce their duration.
My only gripe with the Hand spells are their 30 yard range vs. 40 yards.