Welcome to the forums!
I'm working on an nearly 100% converted Vampire Counts army and since it's slow going, I've been emjoying the Lore of Undeath because it lets me use my sparse VC troops alongside my numerous LM troops.
I did some number crunching on things to summon
here.
My first game with an Undeath Slann reported
here
I dabbled with a L1 Skink with it in a Tournament seen
here though to be honest, Undeath played a very small part of it.
I believe an Undeath Slann stacks well against Fire, Metal, Shadow, and maybe Life but falls short in utility versus Wandering Deliberations, High Magic, and Light. I believe it beats a Beast Slann.
Playtesting has suggested to me that on a Skink, a level two is best, because you can probably roll a decent summoning spell with two chances. Otherwise you are stuck with the sig spell, which isn't bad but it's not as versatile.
I don't think it's worth buying a bunch of undead models if you didn't have them already.
If I ever play a game with End Times magic rules from
Khaine I definitely plan to take High Magic and swap out for an Undeath spell and the free extra lore mastery.