Actually his rerollable 3+ invulnerable save will save 89% of the time. Obviously that means shooting railguns at him is about the dumbest thing you could do, aside from not being able to target him as an IC... if he's in a squad you'd just be killing guardians or whatever.
I've made plans that use Eldrad or equivalent farseer in a squad of Guardians with conceal warlock, and they would be fortuned. It's a 5+ rerollable cover save for the whole squad, saves 55% of the time. I'd beg my enemy to waste railgun shots on that.
Eldrad basically is garbage in melee but don't forget he has a pistol so he can charge with 3 witchblade attacks, or chuck the spear at vehicles for 9 strength.
Eldrad is a strong support guy, taking him out isn't very essential because he can't get anywhere fast. Unlike the other farseers, he doesn't have fleet, and putting him in a transport is a waste, so he'll always be footslogging it. And his powers have very limited range, guide and fortune are 6" so his mobility is low. There's better things to kill before you attack the farseer-supported heart of the Eldar army. But I can see how you would be annoyed at him if you usually rely on a lot of psychic powers.
Assassins... either way you attack, Eldrad should have his 3++ rerollable... maybe Vindicaire with the rounds that ignore invulnerable saves? But that's only 1 wound. What can i say, eldar are tricky.