What SpardaSon said..
Plus we can't duplicate multi vector attack mode, or saucer separation in Sins (we can, but once a ship separated it must stay that way). Berman, and Braga trek had a fascination with indestructible "Uberships" The Prommie , General Chang's fire while cloaked B'rel, The fire while cloaked, Armed to the teeth, Nigh indestructible Scimitar, plus many more that existed only as a plot device. One could say the Big-E's, and Voyager were examples of the "ubership". Least till 2 movies blew up the refit, and D, and wrecked the Ent-E.
We "try" to keep this kind of absurdity out of the mod. We have very powerful units (especially the Borg units). However, No ship is indestructible, or invulnerable. We keep the "unique" classes out Like the Scimitar, and Prommie. We didn't chose units just because they "looked cool". We chose them because they all served a function. Just like everything on the Big-E served a function.
Not to say that the Prommie wasn't massed produced later like the Defiant was. We just have no canon evidence that there was ever more than one ship (other than alternate timeline/universe crap that we tend to ignore).