This brief post will explain why and how they got quite a boost.
Shadowseers have the 'Veil of Tears' ability. This is found on page 49 of Codex: Eldar, and reads thusly;
A Shadowseer is a psyker and always has the Veil of Tears psychic power. It follows the same rules as Warlock powers (see page 20-21).
The Shadowseer uses her powers to confuse and terrify her foe. Any enemy nit wishing to target the Shadowseer or the unit she is with must roll 2d6x2. This is their spotting distance in inches.
If the models are not within spotting range, they may not fire that turn. The Shadowseer and her unit can always be ignored by the enemy for the purpose of determining target priority.
The FAQ tells us;
Replace the second and third sentences with: "The Shadowseer, and all models in her unit, have the Stealth and Shrouded special rules."
In case that isn't clear, here are the sentences we are instructed to replace, one more time:
It follows the same rules as Warlock powers (see page 20-21).
The Shadowseer uses her powers to confuse and terrify her foe.
So...that's only an errata in the most technical sense. An erroneous errata, if you will. And, believe me, I shall. Anyone fields Harlequins (from either Codex, I'm not a dickhead) against me until this is fixed (assuming, in fairness it is a mistake, not a buff. Not, for instance, Phil Kelly's parting act of Eldar-boosting) against me, they get both the Spotting Distance mechanic, and the +3 to their Cover saves. They're still T3 and I gots me some Flamers, what's the big deal?