The prism cannon has a range of 37 (only) and fires only once in 8 seconds. Other artillery have a range of 80 and reload of 5 secods. The damage of the blast is mostly morale.
It does about 200-330 (265 average) damage per shot with 100% accuracy but has -100% accuracy when moving. This means it can't fire it while moving but it also has a setup of 0 seconds so it fires the instant it stops. Weird. Maybe that causes the weird behaviour of the prism targeting and moving in melee range.
Divided by 8 (reload) the damage is weak. About 33. This is further reduced by penetration % values.Penetration is low (in the twenties) for infantry and good for vehicles, but only building_low is it any good. Other buildings its also low, in the twenties.
So you'd better be happy it's got those other weapons ot it'd take you ages to kill practically anything.
As artillery it isn't that brilliant either being unable to outdistance even anti_vehicles turrets and LPs, and with a slower fire rate. The 100% accuracy is only marginally effective, since artillery is usually fired in masses of enemy anyway. And the prism tend to shoot the cannon at stupid things.
In other words the Fire Prism desperately needs a buff not a nerf :-p
OK I'm just kidding, don't worry I didn't go nuts. I wouldn't say no to a range increase though in Mirage Knight's mod that's really nerfed them good.
/edit: the min range is only 5 which means you can shoot your god damned self, since it has a 10 radius area of effect.