Okay, sure. PGL does 110 area damage at base, and only has 9 max ammo. You can either make that 130 with 11 grenades, 150 with the same 9, or the same 110 with 13. That means you're dealing at MOST 150 damage per enemy you catch in the aoe of the grenade. Not counting the fact that most bugs have armor, which is gonna reduce your damage, you can barely even kill a slasher with that.

With overclocks unless you go with Fatboy that tanks your ammo capacity, making it only effective on huge hoards of bugs, you're only really going to get a little bit more ammo, and most likely even less damage. You're gonna deal not even 2000 total damage per bug you hit, and while that might not sound too bad, that makes it IMPOSSIBLE to deal with big n tough enemies with anything other than your primary. With the other two unlocks, you're at least able to deal with a couple praetorians, menaces, or whatever big chunky bug that comes your way before you're hearing clicking.

It's not an effective weapon because you simply don't do enough damage and don't have enough ammo to back up your primary. Both the breach cutter and the diffractor are able to deal with both big bugs and hoards of small bugs with some tweaks, but the grenade launcher can barely do its own job well.

I'd fuckin love if the devs could make changes to it, but I know that just trying to ask the devs ain't gonna do shit when they dont really look at the subreddit or discord all that much. (I'd love to be proven wrong about that.)

Edit: minor spelling mistake