Originally projected to be getting a massive buff in Patch 12.0.5, Unholy Death Knights have instead dropped down to the very bottom of the Overall DPS charts on WarcraftLogs due to a plethora of bugs.
From the Acherus Death Knight Discord, Taeznak has summarized many of the bugs that are plaguing the spec:
All of the bugs have been confirmed fixed
Acherus Discord - Unholy Channel
Commander of the Dead doesn't always apply to all pets its supposed to.
Dread Plague and Virulent Plague tick at a different time than expected after being extended by ANYTHING
Disease Cloud damage comes from the player, not the abomination, causing it to not scale with many pet damage modifiers
Unholy Devotion is providing haste, attack speed and energy regeneration, causing Timmy (the main pet) to auto, and cast Claw way more than he is supposed to.
Suspected Bugs:
Magus of the Deads seem to stand there AFK for ~1-1.5s after spawning. I'm not sure if this one is consistent or not so, grain of salt.
Forbidden Knowledge seems like its getting ~25% less procs than its supposed to based on N dread plague ticks
There's also another weird thing, but not necessarily a bug as it works different behavior.
The Apex Talent has a Mastery buff that's given whenever Putrefy is cast Forbidden Sacrifice. The tooltip says 3/6% Mastery is given, which generally for Mastery % buffs would expect to be multiplied by the mastery coefficient (IE, Skyfurys tooltip says 2%, but gives 3.6% for unholy). But, for this effect it isn't, its just a flat 3/6%
