For what it's worth (which may not be very much), here's a bit of historical perspective. Of course, history doesn't have to worry about game balance, but I hope it's useful or at least interesting.
Bow vs. Powder: This is a problem in just about every game I've come across. This is because most games model the disadvantages of black-powder weapons (especially the long reload time) but not the disadvantages of bows. It leaves a lot of people wondering how anyone ever thought early guns were a good idea, and why they gradually replaced bows in military use.
Bows like the English longbow take a LONG time to master, as well as constant practice. They require serious physical conditioning, to the point where archers' skeletons from the Mary Rose and elsewhere actually show evidence of asymmetric muscular development from the action of drawing the bow. I've never seen a game reflect this, though (I guess it could be done by making archery a very expensive skill to acquire and upgrade), perhaps because players would revolt against a game that denied them such a basic fantasy-medieval weapon.
Guns, on the other hand - even early black-powder guns - are pretty much point and shoot. I haven't researched the subject, but it seems to me that crossbows replaced bows and guns replaced crossbows not because they were more effective weapons (at least the early ones weren't), but because they required less training and made the resulting troops cheaper and quicker to raise in large numbers. And speaking of troops brings me to Ranged vs. Melee.
In history, ranged and melee were specializations: almost no one was skilled at both, and the equipment for each was very different. Archers were lightly-armored (if armored at all) because they needed freedom of movement to use their bows, and that meant that if they were caught by armored melee troops, they were doomed - their primary defense was running away, and their lack of heavy armor gave them an advantage there - except against cavalry, of course. Most games don't give players ranged combat penalties for wearing heavy armor, again because that's not how fantasy works.
How to deal with the issues that willmanx raised? It's tough, and no game I've ever come across has got it right.
I like the range misfortune dice a lot. I would apply them to guns as well as bows, since early guns were not all that accurate until rifled barrels were developed (which just about tripled reload times because they demanded a snugger fit between round and barrel). I'd also suggest adding another misfortune die for wearing medium armor and 2 for heavy armor.
How to reflect the need for archers to have much more training than gunners? Well (at a very broad, non-crunchy, theoretical level), I would probably make bows far less effective at base skill levels, and allow players to develop proficiency by buying skill training or specializations that actually make the weapon useful. Guns, on the other hand, would be easier to use from the start and have far fewer of these skills and specializations because they are so crude at this level of technology that there's very little skill to be learned.
I'm also a big fan of misfires when dealing with black-powder weapons. It's far from certain that an early gun will actually do what you want it to do: it can hang fire (going off the round AFTER the trigger is pulled, possibly just when someone is trying to clear the barrel); it can fail to go off at all, requiring the barrel to be cleared (and unloading takes a lot longer than loading because you can't pull a shot out with a ramrod); it can generate too little energy to fire the ball but enough heat to weld it into the barrel (which can take days to fix or ruin the gun forever); and in extreme cases it can simply blow up taking the user's hands and forearms with it. Rick Priestley came up with a really great misfires table for WFRP1 - it would be great if someone had the time to convert it to WFRP3.
I hope this wasn't too boring - just some thoughts I've had on these issues at various times, but never had the opportunity to develop into detailed rules and test.