They aren't underrated.
At least the Forge certainly isn't. It currently is one of the better weapons that pack Bubble Blower. The weapon itself is solid, being on the long range spectrum of the shooter class with a solid 3 hit kill. You'll see it.
Maybe less so the original Splattershot Pro.
With the Custom Dualie Squelchers here, that tends to be the more popular weapon for Ink Storm. Same range, has an offensive sub, better mobility. Pro still has its advantages over it, but Dualie Squelchers just seem popular.