@Zandramaas
This is a debate that is over the confusion of the term "unit" and "psyker unit." A "psyker unit" is a unit with the Psyker, Psychic Pilot or Brotherhood of Psykers rules. A seer council of two Farseers and one Warlock Conclave is one "unit," but it remains three distinct "psyker units," two with the "psyker" rule, and one with the "brotherhood of psykers" rule. No one "psyker unit" can cast the same power twice, but multiple psyker units can, provided they each have the power. My Warlock Conclave can't cast Destructor twice, but if both Farseers have Fortune, they can each cast it, as they are not the same "psyker unit."
@Partninja
In my set up, I add another Farseer from a CAD (who doesn't benefit from the formation 3+) to run pure telepathy while the two council farseers rune pure fate. While it's somewhat unlikely that I'll get both Fortune and Invisibility, I almost always have Fortune, and even when I don't have that, I still have a chance at Invisibility. In the case they get neither power, it's true the effectiveness of the unit goes waaaay down, and they usually are taken out with priority by any wise opponent. But that's on foot, and I think jetbikes might mitigate that. Honestly, I don't care if the guy passes all his witchblade saves, because he's not going to hurt my seers, and all he's doing is tying up his best melee unit with a unit he can't hurt. Honestly, when a seer council actually kills something in close combat, it's a bonus. The nice thing is people don't want to send just anything at them, it's usually their hardest unit... and to me it doesn't matter. Fortune on a 4++ means you're going to have a very hard time doing any damage, no matter who you are. The Council's main damage output is powers outside of combat, and singing spears. Eldritch Storm, Psychic Shriek and even the maledictions can just be a center of support for the rest of your forces. Throw Jinx on some Terminators before the Dark Reapers shoot at them, Horrify a Wraithknight before Psychic Shrieking it. Guide yourselves and throw spears at a Heldrake. There's really nothing they can't do.