UE has 2 components - a shield wall style dmg reduction and an immunity to silence/interrupt effects. In theory, that's awesome. In practice, I find myself using it 100% of the time for the interrupt immunity offensively or defensively to peel. I ignore the dmg reduction component entirely because I absolutely need the other effect to do burst dmg (Haunt/MG or CBs) or cast a Fear. In and of itself, that's not the end of the world, but when looking at the broader picture of overall survivbility + ability cooldown/set bonus, we start to run into issues.
UE is a default 3 min CD. That's absolutely huge. Most trinkets/offensive burst abilities are on 2min CD or shorter, meaning UE as a counter-burst mechanism is already too long. Moreover, if you just look at it as an Aura Mastery (which is how most pvp locks will start to view it), a 3min CD is over the top. Even the set bonus is a meager 20 sec reduction, which does nothing to solve the problem. Add in that we already have 2 long-CD defensives (bubble 3min and dark regen/HS 2min) and no short-CD defensives aside from the at best OK Twilight Ward, and it becomes clear to me that UE is trying to do too many things and doesn't succeed well at either of them.
In the current state, most teams I'm playing are doing the 'zomg facetrain the lock' strat. Shocker. This applies to double melee, double casters and mixed comps alike. I'm blowing bubble/DR/HS any time I need to counter burst, but Pillar/Shatter/BM/etc. are all back up before I have another CD ready to make life easier for my healer. Even if I pop UE, it only delays the cycle 1 extra CD rotation. Moreover, I can't do that if I want to put out a kill ever thanks to my instants aside from shadowburn being mediocre.
TLDR - I think both UE and our 2-piece set bonus need to change. We need a silence/interrupt immunity spell on a 2min CD. We need a separate 20-30% dmg reduction spell on a 1-1.5min CD. This can be done in any number of ways, but here's the off the top of my head one. I'm sure there are better ones...
UE - Baseline 25% Damage Reduction, 1.5min CD, usable while CC'ed.
2-set bonus: Your Dark Soul spell also provides 8 sec of interrupt immunity when activated.