Personally, I like them. And with most settings being bland kitchen sink fantasy they're just as easy to incorporate as some primitive bronze age culture.
They don't need to be radically different from other ranged weapons like bows or crossbows. That's some bullshit most fantasy nerds have stuck in their heads that they were incredibly inaccurate or dealt the damage of mini nukes. They had a similar effective range as bows and crossbows.
Like other anons have mentioned, pike and shot formations were just evolutions directly out of the volley tactics of bowmen or crossbowmen.
Potentially it could pierce lower quality or simpler armor. Bulletproof comes from testing that would be done on cuirasses. But a .70 calibre ball could punch through simple armor much like a hardened bodkin arrowhead.
If you wanted to incorporate realism by saying players need to keep their powder dry, consider that bowstrings went slack when they were wet.
In addition, yes they were slow to reload. Iirc the fastest drilling at the relevant time was Mauritz of Nassau who could have a line fire once ready to go every 15 or so seconds. But crossbows are very slow to reload as well, depending on the mechanism of course. A goat's foot is far faster than a windlass.>t. hisfag
Hope this helps some GMs who want to run firearms in their games.