i think that it is a perfect allied attachment more so for Dark eldar for the combo with the Raiders being better..
my only real issue is that once again Eldar have been given a rule set that breaks the mechanics of the game.
run shoot shoot run and now hit and run embark from combat.. i know maybe unlikely to happen but still why is it needed in the first place?
I never liked that only Eldar can mess with standard organization of turns. While that can be argued as fluffy, it should have been dealt with in another way by simply allowing them to move longer distances,run 3+d6. .. well you get the idea.
Harlequins seem to be the glassiest of glass cannons. I had a skim of friends codex and there are good options but there doesn't seem to be any heavy support or expandable troops.
However the army can always burrow things from their Eldar kin so niches can be supplemented.
I would imagine the army runs like Deathwing, very elite, maybe not as expensive but has to be played with tactical precision of a surgeon with very small margin for error and bad dice.