Many argue that it was an overpowered codex that ushered in a shift in the balance of 40k. The centrepiece of this shift, the Wave Serpent enabled Eldar players to take a transport with enough fire support that the army's default tank, the Falcon became redundant.
Key to the success of this vehicle was the Serpent shield. The option to use it in a defensive or offensive capacity is still a bone of contention amongst 40k player of the non Eldar persuasion. The 'ignores cover' attribute was particularly painful for enemy vehicles, negating 'jink' saves just added to their fear factor.
If the Wave Serpent was king of the transports then Wraithknight was its wing man. Its presence on the table normally serves as a distraction. These two still are the staple of my force. But I do not take them because they are formidable units. I take them because they enable the rest of the force to work.
The biggest weakness of the Eldar is their toughness or lack thereof. Walking Eldar infantry across a battle field is a recipe for disaster. They will never make it into combat. All Eldar infantry has a range of 24 or less, outside of Dark Reapers. This means almost every other army can shoot the Eldar before being shot at.
The Wraithknight acts as a bullet magnet and the Wave Serpent keeps all the little guys safe until it's time to deliver their payload. The Eldar get none of the cheese like the Necrons with their save and reanimation protocol or the Daemons with their invulnerable saves t. Space marines have infantry with a 2+ save and 5+ invulnerable save. If I take Wraithblades I can give them ghost axes and force shields and they will have 3+ armour and 4+ invuls. But they have no delivery method. No assault vehicle to charge from.
The Howling Banshees were a once formidable unit. But the lack of a an assault vehicle means they are not fit for purpose anymore. And their aspect warrior kin Striking Scorpions suffer a similar fate of not quite as bad due to their better armour and stealth.
Still I suspect the Eldar to be hit hard with nerfs. I think laser lock, the ability to twin link weapons after hitting with a scatter laser will go.