UH’s AoE is immobile but very powerful. You just need to have your tank not to move out of DnD because it’s one of the gripes we have as an UH DK.
Frost is strong in cleave too and people say Frost is stronger… where Frost doesn’t even have an AoE RP dump unless specced to BoS, GA(why does this have a CD?). But they have strong CDs like RW, Frostwyrm’s Breath, Baby Wyrm too, and Frostscythe spamming will be nerfed in SL and I bet they’ll still be strong.
Unholy has crazy scaling as epidemic does have a ripple effect so go nuts with spreading your personal COVID and make them puke.
Take your pick and play whichever the spec suits you most. If you’re having fun with a spec, stick to it. That’s what ultimately matters. I like how creative people can be with Frost. Icecap or BoS, etc. To me, both Frost and Unholy seem competitive in terms of DPS and neither of them should be underestimated.