The only rhino advantage with a mythic set is that he has the quicker first combo, charge 12M and redem.
Problem comes when weapon has too much dmg it can nuke with 2 skills. If sets get a dmg nerf, rhinos wont be nuking with first combo, after combo all they do is auto attack.

Think what you are asking before it.

Rhino is weak but set dmg make them nuke with 2 skills.

Maybe nerfing dmg is not a good idea, adding more armor or health could be enough.