And finally, just finished a run from 1 to 20 using the Abyss Pact for warlock. I try to run everything on r1 to r4 depending on the quest. My focus at m is racial past lives (I'm not a completionist of anything and I only have a handful of reaper points).
My first impressions of Carceri and Celestial can be found in the hyperlinks in this sentence.
Undead are a pita, and are very common. Thankfully you don't heal them, but the immunity still cuts your dps significantly. Single target SLAs are good for orange and red named enemies, but do little when a hoard of a dozen undead are swarming you. Coming off of Carceri, it was nice to be able to DPS mephits and oozes again, BUT undead are not the only things immune to negative damage. Most Constructs are also immune. Also, any enemy caster who can cast Deathward (Blood Tide Spirit Walker's for example) make themselves and enemies near them immune as well. The Cunning Gelatinous Cube is also immune to negative energy damage.
The few constructs who are not immune (that I fought in my 1-20 journey this life) are: Animated Arms, Harmful Thoughts, Iron Defenders, and Multiplication Tables. All other constructs (animated or static) are immune. Most static constructs (alters in the Fire Caves in 3bc, Ballista, various crystals, shroud portals, etc) are also immune to Utterdark's Evil damage (though they still take the chaotic damage from Consume).
As such low and even mid heroics were painful. In Ravenloft there are lots of undead and lots of constructs (scarecrows are constructs and are thus immune). I actually had to lower the difficulty I normally run ravenloft at in order to maintain the pace I normally have, and I still used more shrines than usual due to killing enemies still taking longer and supplementing with tentacles often.
Halt Undead is... interesting? On paper it sounds like it'll help alot with the undead problem. Taking extra time to kill them due to essentially half damage vs them isn't bad if they are immobile and don't hit back... problem is, it isn't clear what undead are smart and thus get a save. Smart undead I found often do make their saves, and when they don't, you only get one free shot on them before they become active again. Non-intelligent undead don't et a save and stay dazed even when damaged for the duration of the spell, but apparently non-intelligent undead are few and far between. There have been many times where a non-descript skeleton or zombie was hit by Halt Undead and they got a save, and if they failed they became active right away again as if they were intelligent.
Abyss also suffers a bit of what Fey suffers, in that they have to split their skill points into two different skills in order to maximize their DPS potential. Negative Energy uses Heal, not Spellcraft (just like Sonic uses Perform, not spellcraft). With Fey it hurts, but not as badly due to almost nothing being outright immune to sonic damage. Abyss already has a huge number of enemies immune to it's damage. Fortunately for Abyss, it does have a belt that grants an Enhancement bonus to negative spell power (sonic does not). However, There are also limited options for Abyss warlock items for negative spell power (at least in low and mid heroics). Unless you farmed the clubs from the anniversary party, your options are limited. the ML 2 KotB weapon until (IF) you farm up some Slavers gear, then the ML 10 Barovian Scepter (which will be your source of Insightful spellpower as the ML 14 Orb that grants insightful negative spellpower is wizard exclusive and UMD does not let you bypass that restriction like it should... I used the Barovian Scepter all the way to 20 because there was nothing better).
At level 14 you gain Death Aura... which for level 14 is almost a waste of SP. the extra damage is relatively small and only in very close range to you. Once you get undead form at 15, then it becomes quite useful. The constant healing should your temp HP ever get depleted is nice. Unfortunately, Abyss Warlocks do not gain any burst negative spells for emergency healing. Like when a champion or Carnage reaper hits you for several hundred in one hit, the ticks of Death Aura will not be enough to undo that/save you.
In the high heroics, Abyss pact looks to work better as a warlock tank than DPS. Especially at 17+ when you can use the Dreadkeepers set. Normally Fiend Pact warlock tanks go for flamescleaned at 15, but the spellpower of that set lines up for them. Abyss don't have a medium armor set that lines up with their spellpower until Dreadkeepers at 17. Damage is still relatively slow, but you become much more survivable, especially with Enlightened Spirit investment. Not being able to use Celestial Spirit and Undead form at the same time is annoying, and means your DPS and hate-threat generation is lower than a Fiend Pact warlock tank, but it's still respectable and gains a bit more regular self healing via Death Aura. Of course, dreadkeeper is negative and force, so to keep Utterdark powerful you'll need to mix in the Hallowed Trail and Respendant Fury.
Rend the Soul, gained at 19, is really powerful so long as you're not fighting undead or constructs. I used it as extra Boss DPS, and it was a huge boon (especially after switching to and ES tank at 17). It's a shame it's cooldown is longer than it's duration, so maintaining it constantly is not perfectly possible, and stacking it (or hitting more than one enemy at a time with it) is impossible.
So in the end, I can see potential for Abyss Pact warlock tanks in late heroics and possibly epics. But for heroic leveling... no thanks.