Slaanesh is arguably the WORST mark you could give them, completely useless in all combats against anything not I3/4/5, including any vehicles - expensive waste of points, unless your meta contains nothing but marines.
Just not as good as other options already elsewhere in the dex IMO
Mutilator pluses are : varied weapons, fear (wow!) and a single extra wound
Negatives vs similar (ie termie) : low LD 8, no guns whatsoever, can't run, get expensive if you try to protect them from instakill with MoN, max of 3, FAR LESS attacks than the termies, vulnerable to force weapons and s8-10, zero flexibility outside of cc in weapon loadout. Most importantly - no dedicated transport raider, so it costs you an elite and a HS choice cause anything other than a raider and they pretty much are not getting into CC. I NEVER have spare HS choices in any of my CSM lists, multiple packs of oblits / maulers are almost auto picks, and havocs and vindicators are occasional (rarely have slots spare!)
I'm not willing to pay 30% extra on top of terminators (which are still over costed at that price!) just so I get to pick a cc weapon. Bring terminators with a selection and use the other bodies as shields for the weapons you need to get into combat