So I've been thinking about this. A lot of people (fairly) point out the early modern / gunpowder era is under-represented in "historical fantasy" settings, and they're right. Or, well, almost. The typical fantasy setting "right now" is often very Renaissance / early modern in aesthetics and tech, just with the guns surgically removed.
I did a good bit of reading (thanks r/AskHistorians reading list) trying to build my fundamentals, thinking I'd try my hand at including the neglected guns... and oof. I don't like them.
So this isn't a "hot take". Right now at least I'm writing to support the popular position. I now get why guns are unpopular.
But Warhammer...
...gets guns wrong. While WH Old World fantasy doesn't quite reach 40K levels of an officer with a bionic eye brandishing a Napoleonic cavalry saber while riding a WWII-type tank to lead a charge of supermen with laser pistols, WH is still obviously more about "fun kitchen sink" than seeing the consequences of its technological developments through.
So, what are my problems?
1. the "mixed stage" where melee coexisted with guns was fundamentally unstable
Hand-held small arms, the "arquebus" started spreading widely in South Germany by 1450, early attempts to integrate them into massed infantry troops were made by the the 1470's (with mixed results), but by the 1500's to 1520's Italian Wars they became the main source of death on the battlefield.
I don't want to write an essay on 16th century warfare, but the tl;dr is, guns reached a "stable" state early on; by 1530, guns reached a mostly-finished form they would retain until the 19th century. I say this because initially I hoped to "stop" gun development at the moment I found best to conserve variety-but that turned out to be (mostly) illusion. Warfare changed due to experiments and culture and growing resources, but there is no part of the arquebus that can be removed to make it... less overwhelming.
The brief window in which Western European warfare was a glorious mess of new and old - very roughtly between the 1500's and 1540's - wasn't a situation that could go on indefinitely. Slowly, commanders realized what worked and what didn't, and field armies gravitated to just being men with guns.
For all the flack "medieval stasis" gets, for me one of the joys of fantasy as a genre is being relatively timeless: this doesn't presuppose stasis, but it does mean that characters and civilizations can interact across timespans that IRL they just don't get to. Fantasy is making sense of the 3,000-year history of Pharaonic Egypt rather than the volatile timespan of European nations re-inventing their core identities every 50 years between 1797 and 1991.
(the elf-meets-time-warp thing is definitely a thing... has already been done... not my main course)
I like to be rational and consistent with the material background of my world(s): and once guns are introduced, unless they vanish for some reason, they will see their logic through. No field of warfare could remain in the "glorious mess" stage of the early 16th century forever.
*(in fact, there's reason to think Europe stayed there for longer than necessary - partly because the Swiss pikes defeating the Burgundian handgun-curious army made a lot of people think small arms fire wasn't a promising direction of development for another 50 years)
2. guns are the point where machines become more important than people
The main problem. What people from non-Western societies down to the 1800's kept remarking about the European mode of drill-and-volley-fire fighting was that soldiers became more automatons than people.
Guns ended the era when culture or personal skill were the determining factors in warfare. Young men could be turned into competent gun-soldiers in a few weeks or months, turning quantity ("manpower") into the only real resource. Whereas f.ex. longbowmen or knights were distinguished by a life skill - the specific arms were part of the person, the same way the longbow or horse were part of the social class, Welsh yeomen or French gentry - raw recruits were made to use the gun by a mechanical process that churned out soldiers out of 10,000's of (mostly) urban young men. People were just the raw material remolded to fit the tools and the job - a sentiment I've seen echoed about the soldier down to modern times.
A musketry soldier wasn't encouraged to show the old traits of heroism or bravado, even as part of a group. The "new soldier" was expected to learn a series of mechanical movements to handle his equipment, to keep up a steady rate of fire under extreme stress at the expense of all other considerations (volume over accuracy was the defining trait of early modern warfare - you just stood and fired in a general direction). The force exerted by the mechanism was independent of any personal trait: a man's job was to stand and pull the trigger as regularly as possible.
I think it's extremely obvious why the martial ideals of the pre-gunpowder era were bemoaned in their passing, and kept fascinating audiences centuries after. If the fantasy genre stresses the personal over the mechanical or the impersonal - as, to me, it must - I start to get why it repeatedly creates settings where the military forces given shape by gun warfare are minimized.
(I always saw "artillery magic" as a cop-out: if you wanna have mages as ersatz cannons, just put guns in the fantasy world, no big deal. Now that I'm actually giving it thought, I get it. Magic-as-artillery is a way to put the flash-bang-boom into a fantasy world while securely tying it to a force that can only ever be personal.)
If anyone has thoughts (or answers these questions differently in their own setting), I'd love to hear them.