@Geurjakha - it depends on the type of enemy you're facing. Guns are pretty good against goblin hordes and human adventurers, but not strong enough against dragons (unless you can power them up with fantasy materials), and not reliable enough against spirits that can re-define laws of physics in their domain. I also expect them to fail against slimes and elementals.
Damage type can be varied to a certain degree, depending on the kind if bullets you're using. You can load wooden stakes into a shotgun if you have to fight vampires, or silver bullets against werewolves.
So I'd say they are a good main weapon for your undead hordes, but you want to stock up on countermeasures to things that aren't easily killed by firearms.