My fear is if firearms are added then players wouldn't want to play with barbarians, monks or other classes too focused into hand-to-hand combat. A possible solution could be to add bulletproof enemies what had to be killed "in the old way".
Other point is when the enemies are the shooters and PCs can't because they come from a lower-level civilitation
Even if firearms were allowed the PCs could bet for the magitek version because these don't suffer malfuctions. DM and players could invent low-level spells to cause malfuctions of firearms.
Some players could want firearms from d20 Past.
If enemies can use firearms each one would need its own monster-stats.