The current FOTM specs for M+ are not particularly fun to play nor hard to master. Balance allready got nerfed, so there is no easy/M+ AoE build anymore and ww or affliction are for sure next for downtuning. Rogues got their broken assa spec hotfixed and sub is outscaled as a T20 only spec.
Pushing your rating with the easiest class-spec combo is hardly a challenge.
And PUGs who decline you for your non-FOTM class choice would still decline you for your missing orange 4k rating as a FOTM M+ spec.
Honestly I don't push and just do easy 15-20 keys for some extra TF-gear and I don't care for class or spec from random players, but for how many dungeons they did. FOTM ww's/warlocks/whatever with a low dungeon count and high rating (VULCANIC/SANGUINE HOLYDAY PUSHER) are the first I will decline in my runs.