They have lower stats than other monster classes, they are a monster class which makes them automatically inferior to human classes because of weapon proficiency, and they explode when thrown which is a bit of a nuisance, especially in the item world.

Nothing is "bad" in Disgaea tho, any class is usable, for story or post game. Prinnies might need a bit more work and make the game slightly harder but if you wanna have one or two on your team there's nothing wrong with that.

The most Usage i can think of a Prinnie is by giving him 3 Muscle Items and a Ton of Hp+ Innocents to create one deathly Bomb.

Alternatively for IW Runs 2 Muscle Items+ a Weight so that the Prinny get resurrected with 1 Hp on the next Floor. Assuming that works if the "Victim" died trough uh assisted Suicide.

Being able to 100% deal X Damage is quite a useful Tool if you for a Example fuse a lot of Enemys and wants to weaken said Enemy enough that someone in your party can get the XP.

Prinnies are easily the funniest Monster Class to use. Personally i think Monsters bigWweakness is not the missing Weapon Mastery which doesn't matter at Lv.9999 anymore anyway. Instead their inability to grab and throw Stuff really hurts them.

Which is sadly something NIS never changed.

Personally i always use Monsters in my Team since the learn unique Skills which especially in the IW can come really in handy.