Umbreon doesn't get a decent STAB move, Foul Play/Payback are unreliable ingame. Dark Pulse comes from 60 special attack which is even lower than its attack of 65. As for coverage, you have Iron Tail (eww), Psychic (not the greatest coverage with dark), and no real useful support move. There's plenty of walls that do a much, much better job ingame because they can actually do some damage in the process or get unique moves. Your best bet at damaging stuff is Toxic which comes lategame, and you're forced to abuse healing items or Moonlight.
Competitively speaking, it has a lot of advantages (using Foul Play on an EV trained physical attacker always hurts, you get some useful egg moves like Heal Bell or Wish to stall with protect) but ingame? It's slow, doesn't hurt, lacks coverage and by the time you get Toxic, most bosses will have full restores with them negating your damage within a couple turns unless you can KO them fast enough, which you most likely won't. Not to mention the amount of Steel and Poison types you will face when dealing with team Plasma. It does ok with the psychic and ghost E4, but that can be said for a LOT of pokes.
It could be argued that it's more Low than Bottom, but does it really change anything? D: