Having your entire army in Wave Serpents except for one unit (the Harlequins) is just begging the opponent to concentrate all the anti-infantry firepower that can't shoot anything else onto the Harlequins. This is somewhat alleviated by the Shadowseer's psychic power, but I'd still watch out.
Other than that ... it's small, elite and looks dangerous. I'm not too familiar with Eldar, but it looks solid.
