I agree that with Unnatural Ag, the players would play the Eldar just as the Eldar is supposed to be played, but I'm more concerned about the possible challenges these characters could face and actually feel difficult to complete. Like, you can't throw Genestealers at the characters in every adventure…
I'm sure it can be done. It's like thinking about challenges for deathwatch squad… the power level has nothing to do with the level of an IG squad, but you can think of possible enemies/missions/surprises to challenge your players.
Some of examples that come to my mind:
- The Eldar have to protect someone/something (a cripled or slow ally, a relic, a temple). Its obvious that if the PCs are attacked, they can't flee and abandon their positions!!
- The Eldar have to hunt/attack/destroy someone/something. Again, you have to face your enemy and attack him, can't simply run away if your objective involves combat.
- An horde of soldiers/critters surrounds the Eldar characters. Running away becomes more difficult, as does avoiding enemy attacks…
- ¡Fast and agile enemies! From tyranids to other Eldar (especially Dark Eldar, of course).
- Make them operate in thight/closed spaces (tunnels, sealed chambers, etc.).
- Shoot them from reeeeeaally far away (sneaky enemy snipers or long-range weapons). This works especially when opposing the PCs to opponents with a better stealth than them (Space Marine Explorers, imperial Assassins) or when placing the players in the open (a field without cover).
I pesonaly wouldn't enjoy a game with Eldar PCs because I prefer the grittiness of low power characters, but I'm sure it can be done!