This simply isn’t true, except the first sentence.
Frost does get trashed in the forums, usually because of playstyle rather than numbers though. Or that’s what i’ve observed.
I can’t speak for raiding, haven’t looked at the data, but for m+ frost is rated as top tier on the website that does statistical analysis of all +16 and above keys. (Search for m+ tier list all affixes subcreation). Great resource, no bias, pure statistical analysis of each spec and roles performance in keys. Frost is ranked up there with havoc and outlaw for melee dps.
But you’re just outright wrong about unholy needing 25+ mobs to do great AOE damage. Unholy starts surpassing every spec in the game on AOE burst at like 5 targets. Even on 3-4 it’s got good burst when done properly.
A good unholy will burst a pack of 6-7 mobs for 500k dps, I do it all the time, and that’s quite a reasonable size to pull in most dungeons and key levels.
Unholy is weak in the following situations. Without CD’s, or on 2 targets. It’s average on bosses without army and hero (its great with them). It’s weak on very long single target fights. It’s also very weak on short fights, like trash in low keys that dies as soon as you touch it. There are some tricks to play around it, but for the most part it needs its setup time.
Unholy is amazing at medium or large sized pulls, especially when played with a tank who can really maximize your dps by pulling big when you’ve got CD’s. It’s got amazing single target burst with army and hero, some of the best in the game.
The strength of the icecap m+ build is its solid damage all the time. You basically have permanent pillar of frost. It won’t hit the same highs as unholy, but it won’t hit the same lows either. Its easy to play, doesn’t need complex setups and doesn’t need anyone in the group to play around it. Its good dps on every boss in the dungeon rather than great on 2-3 and just ok on the others.
And perhaps the largest thing of all as to why its rated so highly on the stat analysis is that in the really high keys (well over +20) the pulls are smaller. When most pulls are 2, 3, 4 mobs it simply doesn’t favor unholy.
For me, unholy sims and performs better in all situations. I don’t have the gear for frost. I’m keen to give it a go when i get the right traits and gear, and buy a bunch of crit corrupt. Will have my 8x 12% haste corrupts for unholy next time the vendor pops, so ill save for the crit ones now for frost.