The postgame DLC maps are the 10 contained in LvD, UT, and FoD, containing seven of the 17 DLC characters.
Their usefulness varies. Can't speak about Hard, but I used Leaf, Alm, Celice, Elincia, and Eltshan for some period of time on my Lunatic run, and while they definitely had issues, they were nonetheless better than most characters.