Allot of assault units in dark eldar are one and done hammers that will obliterate their target, then promptly die to counter fire. It could be said of grotesques that that label does not apply to them lets look.
There's allot to like about this unit, at t5, 4w with a 5+fnp and PFP they are sturdy for a DE unit, unlike allot of our assault options a reasonable sized unit of grotesques won't simply melt to small arms fire. Offensively they have options, lets get it out of the way to start with, DT liquefiers on a big unit of grotesques is inefficient, while the unit is tough, hard hitting and completely unattractive to charge it is expensive and you would be better off going for more wracks in boats.
Melee wise you for only 1 real option with grotesques even though they have 2 different weapons, unpopular opinion incoming "flesh gauntlets are bad" they have no ap, damage 1 and mortal wound on a 6 the +1 str is just no worth it. Monstrous cleaver on the other hand is -2ap, damage 2 and +1 attack, its the clear winner vs basically everything this should be your weapon of choice against anything that isn't t3 and naked.
For coven you are spoiled for choice practically everything is good for these hulkish brutes, I think artists of flesh might be my favorite -1 damage is seriously strong as it grants resistance to things like heavy bolters, instead of loosing a grotesque to a HB now it just tickled him a bit.
Experimental creations and hungry for flesh makes the unit more reliable when it charges and being str 6 does allot for the units killing power vs things like sisters or gravis marines, basically all the goodies that apply to talos apply to these guys. Being t5 though their use from profits of flesh is limited, strength 6 and 7 hits only wound on 4+ and healing one wound a turn is soso.
Things I dislike about the unit, well they are expensive and they only have 1 model, so if you want to field an entire unit you will have them all there synchronized dabbing and looking stupid (this is one of the most off putting things about them, seriously who decided one sculpt only?) so unless you are prepared and skilled enough to convert some stuff be ready for that.
At the end of the day they are a good elite assault unit for coven, not quite as hard hitting as incubi but definitely more survivable. They would be an auto include if I played coven.
Next time Court of the archon.