Why isn't Umbreon used more often? It has 394 max HP, 350 max Defense and 394 max SpDef, semi-reliable recovery in Wish (and IMO, Wish is a much better recovery move for the Curse set than a Recover-clone could be), Curse to boost its likely-uninvested Defense (and Attack, so you can do a bit of damage) with Baton Pass to pass the Curse boosts to something holding a White Herb, and Inner Focus to stop Haxrachi and Togekiss. Dark typing isn't that bad defensively, although it's weak to Conk and Scizor (and Tyranitar's Superpower), which sucks. It's basically a Dark-type Cresselia, which is good IMO.
Umbreon may have high defensive stats but that can't save it from the fact that its just plain outclassed by the wall commonly seen in OU due to their higher stats, better movepools, and superior typing. For example, Blissey is a better special wall due to its higher Sp. Def and incredible HP as is Jirachi thanks to its much better typing. Meanwhile, Ferrothorn just works much better as a wall in general seeing how it resists the same Dark/Ghost-type attacks that Umbreon does while gaining several key resistances and only having one extra weakness which can be easily covered with the right teammates. Overall, there's nothing that Umbreon can do which these walls can't do a much better job of outside of using Baton Pass.