That always felt like Unholy to me. At least if you are a tank it is.
Frost is a little hard to construct, I guess, but not that much. Really, you just have to choose if you want Unbreakable Armor or a 3rd point in BCB, and decide if Chill of the Grave is better or worse than points in KM (which is determined by how often you find yourself wanting RP. Actually, it's pretty straight forward, at least in PvE.
Unholy, on the other hand... It's nothing BUT sacrifices. You could easily spend
60-61 points in the tree. So, if you aren't using a sim to optimize your DpS, it is way more annoying to eyeball the DpS than Frost (though the latter has some randomness due to KM).
I dunno, I would rate the "difficulty" of designing specs as Blood>2-hand Frost>DW Frost>Unholy.
For tanking, it would be 2-hand Frost>Blood>DW Frost>Unholy.
The last two may be reversed with tanking--I can't decide.