Since reiteration is the word of the day, I'll join the chorus. It's not harder. It's not easier. It's just different. Of course for a shitty game that has 20 copy-paste levels it might be easier, but I doubt anybody here in this discussion was thinking of such games. As long as you have the models and the engine support, even a beginner mission developer can build missions that can allow or even encourage various types of weapons. Who knows, it might actually be easier since different weapons assume different enemies or conditions, and those could be easier to add than to make a more monotonous environment, bestiary, and objectives interesting.