I think they are plenty competitive, in my experience you just need Morrsarr as a hammer/scalpel to deal with some of the BS this edition - ie Slann and necromancers sitting back and generating free units. I've been running fairly balanced lists between 1k and 1.5k and have yet to lose a game having played Slaanesh Daemons, new stormcast, nighthaunt, seraphon, sylvaneth, and Legions of Nagash. Morrsarr have been MVPs every game.
Idoneth are very much a finesse army where everything needs to be in the right place at the right time, and are not nearly as forgiving as, say, SCE or LoN.