You are overstating the threat level a bit. Grenades are weapon 4. This means somebody with supernatural toughness could easily take 10 average shots before going down or 6 really well aimed shots from a grenade launcher. A lot better than the dozen shots to the face needed from a handgun.
True to a point. It depends on what creature you're facing and what the gun rules for the full-auto grenade launcher would be. Something that pretty much every single person on this board forgets when weapons like this are mentioned (see the Heavy Weapons thread and the discussion about the GAU-8) is that these things are fully automatic. As in, firing several projectiles per second fully automatic. If you were going up against a Black Court Master, sure, it wouldn't do you a damn bit of good. But if, say, you were going up against a few werewolves, one or two RCVs, a couple of smaller fae. . . then it would put you on a much more even footing.
Now, it can be argued that something like that isn't necessary when you could just use a .45 with iron/silver/insert catch here bullets. That's absolutely true, but it's not the only option. Nor is making a new stunt to make said gun more powerful. I just wanted to point out that there are other ways to make himself even with the bad guys, if pistolero wasn't his style.
One final note. . . what the hell kind of campaign would have beginners (even Submerged beginners) going up against the likes of a sidhe troll or black court master? A bit of an edge is more than sufficient for the beginning of a game, when (Unless you have one INTERESTING story) you'll be fighting things that are just a bit more than human.
