scarab is a VERY good unit, except people use it as a main line unit when it should be shooting from behind a larger group of units (wraths, your partners *** (4 scrabs in 2v2 is dumb ass move)) its designed to be very powerful and take a few hits but not a 2 unit army...its soft counter to all...its like 13 tanks (max pop) vs 25 banshees (max pop again), the tanks will lose because of the soft counter air has in the rock-papper-sciscors halo wars is built around.
Scarabs are NOT a kill all alone force....