Russell needs another burner to truly shine. Zagrog doesnt need(and wont really accept) any help to do what he does. Maybe a wilbur/g. Falcon for def down. I maxed russell but dont use him very often because he also needs heavy embleming.
I have both Russell and Zagrog. currently I use Russell more ofter because I have GM to pair him with. I agree with @e-man and @C.Occam that Russell by his own will not shine. I won’t replace Ferant with a single Russell, unless its against C.Kadilen,hanitra,or Margareth
Zagrog will shine against a lot of defense who use counterattack, or tank with defense up buff. Zagrog can shine brighter if you combine him with defense down heroes, since he resist attack buff.