It still requires an relatively long time to learn in real-life terms - and I can say for a fact that the operation of a musket, no matter how simple or crude it is, is likely still more complex than most any LOTC magic's usage. Sure, you can simplify their RP, but considering that they are basically a finger of god that feels rather powergamey.
You're right, except pointing out the elephant in the room doesn't stop it from existing. Firearms are still too powerful and complex for the average player to be trusted to use without causing more harm than good, in my own opinion. If something has more red lines than benefits there's little reason to let it exist.
My biggest argument against firearms is that they are useless. Apart from in pitched land battles / sieges or maritime conflicts, firearms like muskets and pistols were very rarely used. The only way to make firearms feasible in any form of combat is to allow either semi or full pre-loading, and that's just opening another can of worms, as was seen with preloaded pistol crossbows which got swiftly banned. They are needlessly complex weapons for a server like this (never mind the fact that lore would need to be retconned to allow their existence) and will just complicate combat RP even further than it already is (which is at level that most people cannot engage in without argument or disagreement).
This only only made worse by the fact that firearms are extremely repetitive and uncreative to use in combat, no matter how much effort you put into it. This isn't just conjecture, it happened in the past with the White Rose's arbalest fetish - every time they were used in combat it was exactly the same RP - not for lack or want of creativity, oh no, it's simply because every time you use a weapon like that it's quite literally identical RP, the only difference is in who or what you're shooting.
Except that firearms brought about the fall of both the 'Castle Age' and completely changed the face of warfare. Yea, no biggy.
Cannons are extremely strong. Consider that a typical trebuchet could fire a large stone ball somewhere in the ballpark of 300 yards with pretty bad accuracy, then consider that a cannon could fire a slightly smaller but metal ball anywhere from 400 to 3000 yards with relatively phenomenal accuracy for the time. This is extremely generalized statistics but it gives you an idea of why castles simply ceased to exist outside of Asia after the 15th century.
Also, I like firearms a lot. I roleplayed using muskets back during either Asulon or Anthos, and roleplayed golden staves as a form of gun later on. I do enjoy them but I simply think they're unfeasible.