Accuracy is the same as any other marker, fire rate is slower, by playing with a pump you are putting yourself at a disadvantage to people shooting semi / ramping. The only slight advantage you get is that you may get underestimated by other players.
They force you to think a bit more about your game basically.
Technically they are inferior in every way to semi / ramping but I personally find them much more satisfying to play with.
Edit - and Tom beat me to it.