Ok, some of you may know that Slaanesh is my least favorite Chaos god, fluff and rules. His/her/it's units clash with my playstyle in every way. They're fast, sure, but they're mind-numbingly shallow, and I believe massed rending is far too unreliable to beat out massed power weapons. They rely on hitting first, and killing the opponent entirely, but with their poor strength (and, you know, rending instead of power weapons) they simply can never pull it off. They usually kill about half of the opponent, then whatever lived murders their face off during a few more rounds of combat.
pictured: more threatening than Seekers.

Oh, I hear some people saying "but Fiends are pretty good! They're fast, tough-ish, and they can waste tanks!" what else, imaginary crowd? 
"And they kill elite infantry! Because they can hit before the opponent, and do tons of damage with loads of attacks!" Before they get murdered, of course. "Oh, yeah." You sure we aren't talking about Flamers?

"Huh?" Yep. Flamers. Think about it. They're fast, can kill tanks from further away than fiends without having to rely on Run! rolls. They can kill GEQ (guardsmen equivalents) infantry from further away than Fiends, with Warpfire. And finally, they kill elite infantry by striking first with loads of auto-hit power weapon attacks.

Let me explain. Flamers burninate in the shooting phase, which is, in practice, Initiative 11. Each one gets 3-6 attacks, depending on their opponent's arrangement: however, these attacks automatically hit, wound on 4's, and ignore armor. This is vastly superior to whatever Fiends can throw out, or bloodcrushers for that matter. And after that, they die (unlike bloodcrushers) but then again, so do Fiends.

And they can do all this, anti-tank, anti-GEQ, anti-elite, on the turn they drop.

Slaanesh is almost always outclassed by other units. It's topped by anything with Bolt of Tzeentch for anti-tank, and anything Khorne-ey for killing power. Sure, it could be argued that it sort of does 2 things- but the fragility is inexcusable. Lasguns kill them in droves. Any close combat unit worth their points will destroy them in an assault phase (grey knights with halberds come to mind) and they have to rely on 6's to kill even a 3+ armor.

I don't like relying on 6's. 

I hate Slaanesh. Daemons are random and fragile and struggle against vehicles enough, I don't need Slaanesh enhancing all of those problems. If other people use them, fine. Go nuts. They're fairly cheap, (and they ought to be) they're good at rooting units out of cover. But I don't like units that can't go toe-to-toe with the worst realspace has to offer. A warrior that preys on the weak, and can but beg for mercy against the strong, is hardly a warrior at all.

I'll get bloodcrushers, Flamers, Chariots of Tzeentch, Pink Horrors, Soul Grinders, Plaguebearers, and Daemon Princes of Tzeentch. Come at me.