Is the warscythe a power weapon?
Page 14 of the Necron codex, under the warscythe entry. The work similar to power weapons, in that when you make attacks in close combat they deny normal armour saves. However, warscythes have additional rules in regards to their attacks, which can be found in the weapons entry on the mentioned page.
do power weapons work against the wbb roll?
Page 13 of the Necron codex, first line of the third paragraph. If a Necron with WBB is wounded by a weapon that allows no armour save then it cannot attempt we'll be back; the res orb would allow you to bypass this (look at the item's rules for the reason why) but otherwise if a power weapon wounds a necron warrior it cannot come back.