Are you smoking something? It needs to have that as well before you start considering it?
This thing got pumped to gargantuan creature... that means it gets stomp, FnP, poison only on 6+, instant death now only does D3 wounds, split fire...
For a laughable points increase on a model that was already pretty damn sick... and now the weapons are StrD?? All for under 300... You must have a very weird view of "expensive as fuck" or not have the faintest clue what game balance is. Face 3-4 of them in the new formation style the eldar have... See if you still think it's undercosted.
Stomp is a big one, I think, for clearing tarpits (and the occasional "let's remove 3 Wraiths/Hammernators because I rolled a 6!"). Don't forget Strikedown and snipers only wounding it on 6s too, now.
Expensive as fuck for two non-TL, non-blast, non-Ignores Cover Destroyer shots at BS4. Better be a damn good melee unit.
As an I5 melee unit, they'll shred Knights before Knights get to attack, period. For about 80 points cheaper.
Plus we have support platforms and non-scythe wraithguard to think about, which don't have the -1 to the D table... but will only be ignoring armor, not cover. W/r/t D-scythes, yah, looks like. Ignores Cover too, which is important when compared to these other ranged D-weapons, but... don't forget the "D3 wounds per 3+" aspect of it too. Now you're not praying to roll a 6 to wound, but will be splattering any MC without a fantastic invuln basically guaranteed, if you're in range.
I imagine we'll still see some Wave Serpents for pinch shooting and to transport Wraithguard, but... yeah, jetbikes are going to be more prominent.
I feel like they struck a pretty good balance with Wave Serpents, honestly, not over-nerfing but providing some much-needed balance to the unit--from a brief overview, not having playtested, obviously. 2D6 S6 Ignores Cover shots per Wave Serpent is still going to make for one hell of a shooting phase in a pinch. The real question is if it still downgrades pens to glances, and if it continues to do so after the One Use Only.