In the third book, the Undying One starts using the Nexus Arrangement to cut off the Warp influence around the planet. Teleportation throws people through the Warp. Oops.
Fridge Brilliance
Necron teleportation works on principles of physics only understood by a species that hit the limits of science when man was bacteria, and is completely independant of the Warp. They can teleport nice as you please, but nobody else can use FTL.
