I am 50/50 with this, I was in a group that 2 chested a Hall of Valor 12 with meh Affixes. The only reason we got this was because we had a Frost DK, DH and a hunter. All our single target was decent enough to blow up bosses, but we destroyed the dungeon and almost got a 3rd chest because trash was non existent with this group.
I think this is more dependent on the Dungeon and Affixes.
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Problem with Rime is the RNG, it is better single target but not better cleave or AoE. Single target you can use both rime and Scythe procs so comparing the two isn't the best as you will still be getting Rime procs, even if it is less. I would compare more the single target gain for the extra froststrike you get compared to the Rime procs.
Frostscythe is also not far off a total dps loss from RA on single target, it is enough where you wouldn't want it in raid but not enough to counter the importance of AoE/cleave in mythic+. So it isn't about underestimating single target in M+ but looking at the over all picture which causes Frostscythe to be higher dps over all for mythic plus.
That being said, if you are fine without frostscythe in mythic+ then that is fine depending on affixes and group.