Gunpowder weapons are almost more incendiary (pun rather intended) than alignment discussions, when it comes to OSR games.
There's quite a few arguments against them, as well as some general points of discussion, and I'd like to go over a few of them, from my perspective.
1: They feel wrong
This is more or less the "Lord of the rings" defense, and you know? I got nothing to say against that. RPGs are about feeling more than anything else, and if a certain item breaks that feeling for you, that's fine.
2: They are too powerful
This seems somewhat odd to me. Most OSR games are filled with vorpal swords and lightning wands. However, it can be argued that those are limited to only certain character types.
But if we examine this from a mechanical perspective, most rules for gunpowder weapons make them high damage but very slow reloading and low accuracy. Compared to a long bow, it does not really compare well. In AD&D terms, 2 shots at 1D8 damage versus 1 shot at 1D10 is a pretty easy choice.
Of course in a "all weapons does 1D6" system, there's not really much advantage either way.
It can be argued that a gun powder weapon would have great armour penetration, but crossbows already fill this role, and no one is arguing for their removal.
3: Everyone would use them
When firearms advance enough in technology, they'd supplant most other weapons. This is a fair concern, but it is somewhat omitting that D&D has always encouraged this. Long swords and long bows have always been the primary weapons of the game, in AD&D in any event, due to high damage and multiple shots (for the bow).
Replacing one weapon everyone picks, with another everyone picks is not really a detriment to the game (and using D6 damage for all weapons is a better solution to this issue in any event)
4: Very long reload times
It's often tempting to make firearms take 3 rounds or more to reload. Now, this obviously depends on the length of the combat round, but if using the old school 1 minute rounds, a musket could be loaded and fired twice easily in that time frame.
Of course, this is a problem with other ranged weapons as well. A trained archer can do a lot more than 2 arrows in a minute, so maybe the conversation should be entirely different. The problem here is that a realistic ratio of archery to crossbow to musketry attacks would make bows even more dominating than they are already
5: They explode!
I'm not sure if this came from old Warhammer games, but there's a common tendency to have firearms explode on failure rolls. While early gun powder weapons could indeed misfire in a manner of ways, most of those results would be more likely to destroy or jam the weapon than kill the firer.
Of course, bows and crossbows can suffer malfunctions as well, as can melee weapons. How many battles have your hero used the same sword to hack away at all sorts of armoured opponents, alien horrors and acid blooded fiends?
6: They are not medieval
This makes no sense at all. D&D's technology tree spans a very long time. In a game with plate armour and the bewildering variety of polearms, you'd see gun powder weapons appear as well. Most D&D games have no problem with paper, books, glass, time keeping mechanisms and a ton of other things.
Firearms start becoming common in the 1400's, which for most people is definitely medieval.
Of course, if you are deliberately setting the game in an earlier iron age / viking / roman setting, no guns, but then a lot of other things would have to go as well.