Gameplay
Unlike some other armies which have many army choices (squads/vehicles) which work many roles with few specialists, the Eldar have many specialists and few which can handle many roles. For example, the Eldar Howling Banshees are melee specialists and, with their power weapons and other special rules, excel at killing heavily armored infantry units in melee combat, while using them for ranged attacks often leads to failure. In contrast, the Eldar Dark Reaper excels at long range fighting but will almost certainly be taken down in melee by even moderately skilled/equipped melee units. However, Dire Avengers can be outfitted for either melee or ranged combat, though they will not be as able as a more specialized aspect.
Eldar vehicles, unlike their infantry counter-parts, are very tough and hard to kill, pending lucky rolls. While no Eldar vehicle has the maximum armor value (14), they often have upgrades and special abilities which can compensate for this and make them more effective than heavier vehicles. Most Eldar vehicles can also be equipped with weapons designed for various purposes. Some examples include the Brightlance (designed for killing Heavy Tanks), the Star Cannon (designed to kill Heavy Infantry), and the Scatter Laser (designed for killing light infantry). This is in addition to other upgrades, such as the Star Engines, Holo-Fields, and Spirit Stones. With the exception of walkers, all Eldar vehicles are skimmers which allow them to move "freely" across the board and, with upgrades, at speeds only matched by the Dark Eldar and the Tau armies. The drawback is that Eldar vehicles are expensive to field in game. An example is the Eldar Wave Serpent, which is one of the most-expensive dedicated transport vehicles in the game.
Because of the Eldar's comparatively weak armor and high fielding costs but comparatively powerful weapons and fast speed, successful game play is often stylized by outnumbered Eldar units which outmaneuver the opponent and kill entire squads before they have a chance to retaliate. This is also the cause of Eldar game play being regarded as "unforgiving". Unlike Space Marines or Necron, who boast heavy armor, high toughness and some form of wound-resistance; the Eldar do not. Because of this lack of staying power, Eldar infantry is often subject to severe, and sometimes unrecoverable, losses after a bad tactical decision or even a series of poor rolls. They are somewhat comparable to an eggshell with a hammer.
Read more about this topic: Eldar (Warhammer 40,000)