During every previous tier, there was always this one boss that turns out to be completely overtuned on LFR, or where the tactics totally don't fit LFR and most groups wiped at least several times before they were nerfed. So far, these were:
T13: Spine of Deathwing, possibly Ultraxion if your tanks didn't know how to use their EAB;
T14: Garalon, possibly Amber Shaper Unsok if your players really suck at interrupting;
T15: Durumu, Council if you didn't split DPS properly.
T16: Nazgrim, Thok, Garrosh, Paragons, ... basically half the instance.
T17: Iron Maidens, Kromog, although neither were as extreme as the MoP bosses.
I wonder if there will be any boss in T18 that can compete with the MoP bosses in how they are overtuned or a poor fit for LFR, or if they're all going to be as trivial as those in T17.