Grenade launchers have limited application against zombies. Real close-in the fragments have a high probability of entering the skull, but that probability drops below reliable levels the further from center you get. And you can't rely on the explosive shock wave of the blast to take out a zombie brain. It's basically old fashioned penetrating trauma or nothing.
But that doesn't mean grenades and grenade launchers are useless. You can use them to slow down an advancing horde, which means they can be good for breaking contact with the hungry undead. And a repeating grenade launcher like the M32 can lay down a withering barrage of grenades practically on top of each other, so you can guarantee a pretty good kill rate for an expenditure of a large amount of ammunition.
I tend to like the grenade launcher for hunting other creatures, large creatures, like a living dinosaur, or a woolly mammoth, or fishing for Nessie... Plus, grenade launchers can release many different payloads these days, such as gas canisters or flares. Both have their uses.
So I like the M32 and keep one in the toolkit for special purposes.
Sometimes you can't be afraid to dream a little bigger.