I've never hit a vehicle on a 5+, even in 5th edition, where it was either auto-hit, 4+ or 6+.
Because Vehicles (minus walkers) have either weapon skill 1 or two, it makes scarabs incredibly dangerous as, at worst, they hit on a 3+, roll for Entropic Strike on a 4+, then armor pen at the reduced value.
So on a minimum unit of 3 scarabs attacking a moving vehicles:
15 attacks on the charge
15*(2/3) = 10 hits
Entropic Strike = 10 * (1/2) = 5. 5 reduced armor
So against a Land Raider, it now has 9 armor all around and can be glanced to death by the scarabs on good pen rolls.