I agree with you on CSM and necrons, yes. As a very active GK player, I can say without question that the fast attack selections are the worst in the codex (though basically all GK units are at least useable).
I don't think there's anything particularly special about DE fast attack: Reavers are certainly good, but not broken, and Beast packs aren't really Broken on their own...they're broken because of the insane combo you can make with them.
SoB Dominions are good but I think it's hard to argue they're the best things in the codex.
IG sure, Vendettas are undercosted by about 50 pts, but fundamentally that codex is about troops, and not elites.
Still though, most of the really hard-hitting units in 40k are Elites. Paladins, purifiers, harlequins, terminators of all stripes, True-born, sternguard, Crisis suits, Chosen, fire dragons, wolf guard, Hive guard, Nobz, heck, even demon flamers. All the Cult troops of CSM are natively Elites.
Letting Elties score would be a mistake that letting FA and Heavy score is not. And, to boot, many of those units have ways to make them troops through special characters or other special HQs already.
To bring it back off tangent: yes, I think it was thoughtful decision, and I see the reasons behind it.