I'll try and address your question from two perspectives. One, from a medical point of view, and two, from someone who worked at West Marine for a summer.
1: If you go overboard unconscious, you're dead. It's my opinion, but I think there is some good reasoning behind it. If you are unconscious then your only chance of survival is wearing a Type I OR Type II
being in dead flat calm water. Type III, and V PFDs will NOT hold an unconscious person's head above water. All inflatables (that I know of) are USCG certified as Type V PFDs. Some will say "when inflated it performs like a type I" but to be sure, read the label on the device. They wont be USCG certified type I or II, so they will NOT hold you face up when unconscious.
Many many American and British Army troops, seamen and airmen drowned in WWII with foam (or Kapok) vests on while unconscious because they all immediately turned face down. After WWII the British Navy attempted to figure out how to design a PFD that would hold an unconscious victim face up.
Engineering a PFD to hold an unconscious person's head above water is actually very difficult to do. So much so, that someone "acting" unconscious isn't even close to a working model. The only way that they were able to tell how a body would act, and engineer the vests appropriately, was when Dr. E. A. Pask of the British Royal Navy allowed himself to be anesthetized in the water for experimentation purposes.
Now that you know that only type I and II PFDs will keep your head above water, it's also good to know that your typical inflatable will only keep the wearer's mouth 3inches above the water. If you're unconscious wearing our automatic inflatable and, by some act of God, you float with your face out of the water, it'll be 3" above the water. A couple wavelets or the wake of the boat trying to rescue you will push water into your mouth, past your incompetent larynx and you will drown.
From Mustang Survuval, trying to sell their product that holds you higher out of the water:
"In general, inflatable PFD’s are designed to suspend a user’s head above water to prevent drowning. The Inflatable Vest with LIFTTM provides freeboard that far exceeds the minimal 3” generally provided."
Reference Here: http://www.mustangsurvival.com/sites...0Aug2011_0.pdf
2. Now, the other perspective. I worked at West Marine for one summer in order to pillage their discount. And pillage I did....
I kept a mental poll going of replacement cartridges for lifejackets. I asked every single person who I sold a re-arm kit to, "how did it go off? Was it intentional, accidental, or were you unconscious and overboard?"
100% of them were automatic inflatable vests that went off when they didn't want them to.
I have a manual. I don't want it going off when it's not supposed to, and I don't believe in the false security of it keeping me from drowning while unconscious. So since, in my mind, the automatic offers NO benefits, the manual is the better choice because of price. Or does the manual actually have advantages over the automatic?
I considered if the manual has advantages over the automatic. I am a decent swimmer and I don't know if you've tried to swim with those inflable horse collars on, but it's nearly impossible. (everyone who owns one should spend the money on a cartridge and try this). I can imagine scenarios where I might want to swim first, and then inflate if necessary. Another nightmare scenario is where you might be trapped under a capsized boat or raft and have to swim out towards daylight. Or maybe the mast went down and you're overboard and tangled in the lines. I'd want to be able to swim DOWN and free of the boat and rigging and THEN inflate the vest.
These scenarios are extreme to be sure, but then again, so is being hit in the head by the boom AND it knocking you unconscious, AND your unconscious sack of potatoes body going overboard in the process, which is the only scenario I have heard mentioned that makes the automatics supposedly superior.
For further reading, the best book on the subject I have ever come across is "Survival Afloat" by Don Biggs. 1976 ISBN 0-679-50579-2