Well for one thing it is a slow moving projectile and you can't just spray the thing like you can an AR. If you are not skilled with the weapon you're not going to do very well with it.

Also you need to figure out a way to get ammo because as a heavy you can't carry a nanohive. (this sometimes means calling in an LAV to drive back to a supply depot which takes up a good portion of your killing time)

You need to find good vantage points, and you're weak as hell when players are above you.

But yes, in terms of damage output, the forge gun is definitely the best out there. Although I do know the exact secret of what makes the forge gun so effective. I mean, compare it to every other weapon that does a lot of damage. What makes the forge fun different from say the mass driver or the plasma cannon or even the swarm launcher...

It's the projectile speed, and the lack of arch.

No seriously...if other weapons had a faster projectile speed, they would be easier to use and overall more deadly as players could hit more accurately. Swarms aren't that great because the rockets take forever to get to the target. The plasma cannon seems to suck because it has no real use from long distance as the arch AND speed of the plasma ball makes the gun hard to get direct hits with. That's why it's so effective at short range.

The forge gun is so effective because it fires like a regular weapon, except that it fires giant balls that make getting direct hits (instead of doing splash damage) easier.