Well first blackpowder weapons were kind of shite.....longer loading times, less accuracy, less penetration, risk of misfire.
Advantages were ....easy to learn (half an hour of instruction)compared to several years for archery, no strength needed,cheap to produce (the basic handguns was a pipe in which you dropped a lead ball and some powder...bang), during rain the sinews of the crossbow and the bow were unreliable as with the handgun you just needed to keep the powder dry well and the psych factor of the bang, fog etc.
What kind of makes little sense is the mix in the warhammer world of high price (quality manufactured guns maybe even with a rifled barrel) with low effect (low damage and low accuracy).
