These two units fulfill different roles, so it's hard to directly compare them.
Kats are supposed to be used as an area denial artillery, that's why there are 4 salvos. The werfer is supposed to be a very concentrated barage in one go. So wefers are better against blobs, and doing direct damage. It's like the difference between the mg42 and the maxim.
If I'd change anything about the werfer, I'd take a look at it's vet1 ability. Does anyone even use counter barrage? That seems pretty useless to me. Whereas the Kat vet1 ability is very useful, which might be why it seems so much better.
The reason why it seems so much better is the kat can deal massive damage to a blob at max range In the first set of rockets, and if your caught by it your gonna get hit by 2 waves before the retreating infantry are out of it.
in contrast to panzerwerfer's max range all rockets will fall on a area and barely scratch a blob, its not uncommon for when ive fired on a 1919 para or rifle blob all rockets fired on target - 3-8 kills.
The frustrating part is if it was a kat truck it most likely would of squad wiped something and gotten 12+ kills on a sizeable blob.
When I build a kat truck it almost always feels worth it, When I build a panzerwerfer I regret it half the time, Because the fuel would of better been spent on a tank.