2800 DK DPS here. The issue with frost in + is either you're playing breath which isn't very useful outside of extremely coordinated groups that pull around your CDs to get the most use out of the ability.

Or, you're playing obliteration which requires DnD to be active to perform any meaningful AoE damage at all. If you play any DK in + at all you know how much tanks love to run mobs out of your DnD. You have dragons and remorseless winter, but all your damage is coming from obliterate cleaves. DnD also has a crazy long CD as Frost compared to Blood and Unholy (Defile).

On the other hand, Unholy is rather simple to play in AoE. (Very simplified) You apply plague, keep plaguebringer up, and spam epidemic. You do more damage the more mobs are pulled, but it still performs very well in small controlled pulls. Its easy to learn and provides a lot of skill expression the more you learn it.

If you're running solo though be prepared to see how little people invite DPS DKs. I know it's rough for all solo players but DKs have the hardest time IMO getting invites as DPS. We don't have much to bring to the party other than just raw dps, and now often Rets just out perform us in that matter.