I believe that matchlock firearms would misfire more often than that, but they certainly wouldn't explode that often.But in general, the times I've seen people talking about wanting firearms to be dangerously lethal to their user, they're talking about rates of error that're somewhere around 1/36 or more.
Even if they did though, that wouldn't necessarily mean they weren't worth using. You'd have to weight the pros against the cons. A bombard that took down a castle wall might be well worth using with a 1-in-36 chance of killing the crew.