I've had both types. 12v are as reliable as the person that takes care of them. The main reason they go out is because the systems rely on a deep cycle battery to fire the relay that fires the igniter. Some will cheap out and put a car or lawn mower battery in there. Those batteries just don't have the amps for a 12 v system, and in most cases fry the relay or worse.
The only problem we've had with 120v systems is adjusting the belt, but I haven't heard good things about voltmaster generators. If a generator goes out that's $400-600 bucks.