I, too, would not worry about the reliability of auto-inflatable life jackets. There are two types of mechanisms for auto-inflatables; one uses a dissolvable "pill" that will dissolve in a fraction of a second when it gets wet, allowing the CO2 cartrige to be punctured and inflating the jacket. Manufacturers of usually refer to the "pill" as a bobbin. This is the most common type of auto-inflatable, is nearly foolproof, and is cheaper to maintain than the second type which uses hydrostatic pressure to activate the inflation mechanism. Their is only one negative (a small one) that I can think of with this type of inflation mechanism: if not maintained (i.e. change the "pill" or bobbin when the it expires) it will eventually activate on its own as the bobbin slowly dissolves over time in a humid environment. If you are wearing the jacket when this occurs, you will be looking for your spare underwear. It is loud when it goes off without being submersed. Ask how I know this. I wear one of these the entire time that I am on the water. It is lightweight and unobtrusive so I doubt that anyone would have a problem throwing a castnet while wearing one. My wife and I fish approximately 4 days per week and I don't even notice that I am wearing a life jacket, even under the hot desert sun at Lake Amistad where we live.
The second type, already mentioned, is an auto-inflator that uses hydrostatic pressure (4" water column) to activate it. This type of inflator will inflate the life vest when it sinks 4" under water. The obvious advantage to this type of inflation mechanism is that it has to be submersed to a depth of 4" to activate, making it impervious to the effects of humidity or the occasional rain storm. The disadvantage to this type of life jacket is that it is expensive to maintain. The reactivation kit for a hydrostatic inflator is approximately $80-90. Rearming kits for the bobbin type inflator are approximately $25-30. Mustang Survival is the only manufacturer of life jackets that I am aware of that use hydrostatic pressure to activate, though there may be others. My wife wears one of these and it is more sturdy than any other inflatable life jacket that I have seen.
Both types of inflatable life jackets mentioned here have a tube with a check valve in it (on the left shoulder) that allows the user to inflate the jacket from one's lungs in the unlikely event that the auto-inflation mechanism doesn't work. Also, both types usually have a pull cord that allows one to manually puncture the CO2 cartridge if necessary.
USCG requires that conventional (common, orange covered foam types) Type III life jackets provide at least 15.5 pounds of buoyancy. Inflatables are required to provide at least 22.5 pounds of buoyancy and many provide more than that. You will not sink while wearing one.
An important note...A boat operator is required to have one Type I, II, or III for each person on board while underway. An inflatable is classified as a Type V life jacket if you are not wearing it and it classifies as a Type III life jacket while it is being worn. This warning is often on the lifejacket tag as well. In other words, if you want to carry an inflatable life jacket, but not wear it, you must also have another life jacket on board for each person that does this. There is a reason for this... it is nearly impossible to put on an inflatable life jacket after it is already inflated, rendering it no more useful than a Type IV throwable. If you are stopped by law enforcement and only have an inflatable in the boat, yet you are not wearing it, you might very well be fined for not having a Type III life jacket on board unless you have the common orange foam life jackets on board as well. We keep an extra orange foam Type III life jacket in a rod locker for those occasions that my wife likes to sit in the sun and dry off after a swim from the boat.
Another important note.... I suggest buying the pill/bobbin reinflator mechanism in person or from a reputable retailer if buying online. The bobbin has an expiration date printed on it. I was burned one time when I ordered rearming kits from Amazon that arrived with expired bobbins. The kits are easy enough to find at West Marine when I visit Austin, Bass Pro, Academy and etcetera and I am assured of receiving a fresh bobbin when buying in person.
I have spare auto-inflatable life jackets that I keep for visitors. It is not uncommon for West Marine to put their house brand auto-inflatables on sale. The last ones I bought were two for $99! Let us know what you choose.