I have three general points against guns in fantasy role play games.
1. Once they become widespread then you can kill a man, and most other things fairly easily, tipping the balance of power to technology versus magic. (Sure, you could invent a spell that acts as shield or even a targeting displacement, but could you get it up in time before several guys discharge rounds to your head or a vital organ? And don't forget that if everyone or even a sufficient number of individuals have guns then the disadvantages of long re-load and re-fire times can be rather easily overcome by smart and practiced variable rates of fire, along with variable position fire.) So they tip power scales against other forces towards combat oriented technology, but not just combat oriented technology, but technology with a highly lethal potential and capability. Which initially sounds great but this merely redirects the use of magic (as but one easily noticed side-effect) towards ever more combat-oriented expression in an attempt to counter-balance force-projection capabilities (and magic in fantasy games is already far too combat oriented to me - meaning all you really have left is a tactical combat game with magic attempting to match technology at the level of basic engagement, a supposedly game of role-playing would become even more just a tactical skirmish game with new weapon systems). After all if magic cannot effectively compete with technology as a combat tool then it will be abandoned as a combat tool, so as far as combat is concerned magic would either adapt, or die. If you do not believe this then ask yourself this question, "how likely would it be that if gun(s) were introduced into your campaign that this would not be almost immediately followed by an arms-race?" The race implying a rush to technological improvement that would far exceed that of other weapons, because gun weapons are far more open ended and versatile in capability and potential than swords, pikes, spears, long-bows, etc. In the narrow range of combat function they show almost as much potential as magic, and can often be far more directly lethal.
2. How wide spread exactly will they be and how fast will they spread? Once guns become replicable (in the sense of being mass-produced, and believe me a smart political and military power will look for ways to mass produce effective weapon systems) then the period of time in which they will be of little practical value (see fire-rate mastery above) will be a short one. The natural impetus of technological advancement is of relatively short direction and for an easy reason and that is that technology works upon the foundations of science, meaning a thing is almost always replicable. Meaning it spreads like wildfire and improves constantly over the lifespan of the invention, sometimes for generations and generations. With magic, which unfortunately in games far too often mimics pseudo science in effect, the implication (still existing, even if unstated) at least is that magic is very hard to master and so hard to replicate that only a few practitioners or experts may master the principles in any given population group. A child, and I have trained my own children to shoot well, can master any sufficiently advanced firearm and easily dispatch a far more dangerous yet unarmed opponent. If you're being honest about it. (Of course if your firearm is so primitive that it is extremely inaccurate and is very ineffective then that's beyond the point - because why really employ such an inaccurate and dangerous device, except maybe as a psychological weapon, or to render a one time shock effect.) So as a matte of technology and science guns are extremely dangerous to any world in which they are introduced when it comes to replication and how fast they will spread. Because once guns are mass produced they are no longer tactical weapons like swords, they become strategic weapons. Numbers of swords always remain in actual effect, tactical weans. Numbers of guns, behind a good leader, or anyone who really understands their potential, always become over time strategic weapons. And that's a whole nuther kettle of fish. Only in cartoons is Judo ever much of a match for a Peacemaker. Plus it takes a along time to kill a man with a knife if he's fighting back and knows what he's doing, and that kinda thing is extremely dangerous to everybody involved. I can do it almost instantly if he's in effective range of my shotgun. The only real danger to me is the question of how many shells will I discharge before he ain't breathing no more. And that's an economic question, not even much of a tactical one one.
3. I think they defeat the point of personal and heroic valor (only as regards the duel or close combat aspect of fantasy gaming combat styles - I am not implying gun combat cannot be valorous and dangerous and heroic under certain circumstances - I've seen this myself) in fantasy combat. Because of what I said in points one and two above.
Now all of that being said I am not against guns ever appearing in a fantasy setting and a few have in mine. Only one used a modified type of gunpowder, the other three functioned in different ways (one even fired different types of ammunition). But to me guns in fantasy settings should be proprietary artifacts, either created or discovered, have certain built in limitations (all guns, like any other technology, does anyway, it just might not be immediately evident to those unfamiliar with how guns really operate), as well as certain capability advantages, but should not be easily replicable, if at all. They should be like a sufficiently high-powered magical artifact.
That limits how wide-spread they can become in a given fantasy world but does not necessarily limit their overall effectiveness in use for small scale combat.