I see the Keldon class as a separate Cardassian Battleship built to match the Galaxy Class and the Neg'vars. The Keldon was tough enough to withstand a prolonged attack from the USS Defiant so I don't see them as failures.
Why they did not appear later on? Either they where too expensive to build or we just didn't see them in any of the on screen battles.
The problem I have with the Cardassian fleet is that in large scale battles we see dozens if not hundreds of Galors
just sitting in space looking pretty waiting for the Bloody Klingons to blow them up
. I almost take that as a personal insult every time I see that battle scene in SOA
My personal favorite Cardie ships can be seen in the trek Games Amada 2
and Dominion wars,
in particular the Huet(DWS)
and the Kulinor(A2).
The Kulinor: http://gaming.trekcore.com/armada2/s...ardkulinor.jpg
And the Huet: http://gaming.trekcore.com/dominionw...ps/cclass5.jpg