While it really is difficult to find a bad unit in the latest Eldar codex, most of the attention has been on Wraith units (with D weapons), Jetbikes (with max Scatter Lasers or Shuriken Cannons), Wave Serpents, and formations likes the Seer Council or Crimson Death.
One unit which I think has become much more valuable in the current meta is Dark Reapers, particularly if they are taken as part of an Aspect Host (for BS5 shooting). With 48″ S5 AP3 Hvy 2 shooting for each model, they have unique anti-MEW capability out the gate. Add in Slow and Purposeful (for the ability to move and still shoot), Reaper Rangefinders (for the ability to ignore enemy Jink saves), and a special rule that makes their shooting Twin-Linked against anything that Turbo-boosts, moves Flat out, Swoops, or Zooms, and suddenly you have a unit that “counters” many of the competitive units in the game right now:
1 – Ravenwing Bikes/Black Knights or White Scar Biker Armies — With the ability to ignore their 3+ (or even 2+) Jink saves, plus S5 and AP3, Dark Reaper shooting is the ideal counter to a Bike heavy opponent. With BS5 (from Aspect Host formation), a 5 man Dark Reaper squad inflicts an average of 4-5 unsaved wounds against a normal Bike units, which is enough to kill most “typical” bike squads in a single volley, all at a distance of 54″ (6″ move + 48″ range) starting turn 1.
2 – MSU Marine squads — With the shift toward MSU, both due to things like Demi-companies and things like massed D-weapons, many players are fielding lots of 5-6 Marine squads to maximize their damage output and minimize their vulnerability. Dark Reapers can hunt these MSU MEQ squads with ease, allowing Eldar players to “thin” the Power Armor hordes from long-range, even before other Eldar units get inside Shuriken weaponry range to deal with these threats.
3 – Necron Phalanxes and support units — With long range, multiple shots, and AP3, Dark Reapers can attrite down Necron Warriors, Immortals, Flayed Ones, Triarch Praetorians, Deathmarks, Destroyers (all types), and Tomb Blades and allow the rest of the Eldar force to deal the finishing blow to these units.
4 – Tau Crises Battlesuits — With standard MEQ profiles, but multiple wounds and high mobility, Crises Suits are often the “flagship” unit around which Tau battle groups are built. With their long-range, shoot and move ability, and high accuracy, Dark Reapers can focus down threatening Crises Teams. For example, a 5 man Dark Reaper squad gets 10 shots, 8-9 hits, and 5-6 AP3 wounds, which is enough to kill outright (or critically damage, at least) even a full Crises Team, especially if the Reapers can fire from an angle to focus on the Suits and not their protective drones.
5 – Eldar/Dark Eldar/Harlequin Jetbike units — With their ability to ignore Jink and 3+ saves, Dark Reapers are the perfect counter to fighting enemy Eldar jetbike spam (of any variety). Take an Aspect Host of 3 x Dark Reaper squads and they can reliably gun down three Jetbike squads a turn, which means by turn 2-3, you should have eliminated pretty much any MSU Jetbike threat there is out there.
6 – Skimmer Tanks — With high BS, the ability to take Starshot (i.e. S8 AP3) missiles as an upgrade, and ignoring Jinking, Dark Reapers can be a strong counter to Wave Serpents, Raiders/Venoms, Devilfish/Hammerheads, Annihilation Barges/Ghost Arks, Land Speeders, Flyers in Hover mode, etc. etc. They also are half-way decent anti-flyer unit when equipped with Starshot missiles, since they re-roll to hit against any Zooming units/Swooping FMCs, meaning that they have about 30% accuracy against flyers and FMCs even when snap shooting.