Dittos are very powerful pokemon that can transform into other things, something like a living photocopier. The power of the Ditto is great, but not limitless. Dittos may only transform into pokemon it can see /at that moment/, and the same thing for copying a pokemon's voice. Being globs of stuff, they're not terribly bright. Complicated manmade objects are impossible for a Ditto to duplicate. Simple objects are easier to duplicate. While imitating an object, Dittos may only say their own names.
Dittos are also nonviolent by nature. It is not IC for a Ditto to turn into a lethal weapon. Transforming into a cannon on the show resulted in a semi-comical and non-lethal solution to the problem; it was also in self defense. Dittos can't talk like a human, and while with enough experience they can turn into copies of humans, the fact that they tend to not be able to talk ruins the masquerade. Ditto-humans also don't have fingerprints. There is a maximum ceiling on the time a Ditto may assume a form, which is always under a half-hour. After that, they collapse back into ditto-ness, and are unable to do anything until rested. Taking a form is easy. Holding it is not.