Without proper escort, they have no answer for melee units.
Assault mechs and artillery can dish out long range punishment, but have a minimum firing range in which they can aim.
note And speaking of bombers, they also get bonus damage against turrets - including, strangely enough, Anti-air turrets. Naturally, power cores have low HP and bombers get bonus damage against them.
Taking out the power cores will shut down shields and cripple most buildings.