Assuming you luck out and roll box cars instead of snake eyes. You have an equal chance of removing yourself from play or teleporting yourself into CC with the monster you were trying to nuke...
the Overlord is basically a 50/50 chance of totally facerolling who ever he fights, and the other half of the coin is the Overlord gets a good lick in before the nasty character ID's he some way or another.
The Overlord is a hard counter to any nasty character simply because its a roughly 50/50 chance of taking that character out, depending on how durable he is to his own attacks. The Overlord might also get a wound in himself with the Warscythe, and he's durable enough to maybe survive if the character passes the MSS test. But one ID causing wound and he's hoping for RP.
The necron Overlord is extremely good. He will best most ICs in assault. He only makes the target hit themseves d3 times -- so he can still fall to models like Abbadon because when the LD test is passed, Abbadon just brings a ton of attacks to the table. Any unit that does not have the capacity to bring those volume of attacks is pretty much dead.
The weakness of the necron overlord is their ability to only swing 3 times at WS4. This means while an IC like Abbadon or Typhus is erasing squad after squad, the Necron overlord is killing ~2 models a turn. When its engaged with something like a 18 guants, that's just ... horrible.
I know what your thinking "Well, I'll just stick him with 6 wraiths". The reality is removing 12 wounds at a 3++ in the meta of serpents and tau is just not that hard.
Wraiths were the bomb back when everyone was packing melta and plasma. Now that people are bringing tons of high STR weapons that give you normal armor saves -- Wraiths are not as 'hawt' as they once were.
Now you just need luck to not scatter XD
