I have about three years of experience in a server-first raiding guild, raiding 40- and 25-man content 4-5 nights a week, 3-4 hours a night. In fact, the reason I largely stopped raiding was that Frost scaled too poorly to remain viable once we reached Sunwell. It's been a major focus of my feedback in the years since to see that flaw in the game's design corrected, and I'm very pleased that the developers ultimately came around to that point of view as well and have been working to improve the situation.
I'm sure some will argue that experience doesn't count unless it happened recently, but I dismiss this argument with prejudice. Raid mechanics haven't really changed all that much over the years, and if you can't infer from past experience how a spec and abilities will work in new encounters, that's a problem with your ability to draw inferences; not a problem for me.