Well, there is the storage/transport of it, the ease of sabotage, the length of time spent reloading, the chance of misfiring wounding or even kiling the gunner -misfiring was common until the mid-18th century Rifling invention - The short range, the extreme noise, the awkward length of the weapon. 

And I guess taking outdated army would be sort of good, but that would mean using outdated tactics as well, and I like to be kept tactically advantageous.