Personally I think Umbreon is effective in the OU tier. I play OU a lot, and i find umbreon to be very helpful.
massive def and spec def with good Hp
learns payback which doubles in damage after being hit + stab. can learn toxic, which is useful on almost any pokemon.
can learn substitute + moonlight which is great for stalling. when paired with infernape/blaziken and a psychic type its almost unbeatable because the only things umb has trouble killing are bug fighting and steel.