UngeheuerLich
Legend
Yes, of course. That was not my point though. A berserker that attacks a foe that is prone has advantage anyway.I think your missing the extra d10 damage.
2d6+4+4 = 15 *.84 (advantage) = 12.6 damage.
Vs
2d6+4+4+1d10= 20.5 * .6 = 12.3 (+ Push).
Barely any difference in average damage, for the additional effect.
Also worth noting that you can pick each attack. Make one with advantage (get the beserker bonus damage), then use the second to push. Possibly 3rd attack if you TWF, 4 with dual wielder feat.
They still need to reckless attack, or won't get their bonus damage. If they could trade only only the advantage from reckless attack, they would at least get some other benefit out of one attack.
I never said it is totally unbalanced. But I think that would feel better.
Or for other barbarians, just decoupling it from reckless attack would also help, if it would be indeed unbalanced on a barbarian.
So just: on any one attack you can forgo any advantage to brutal strike. So if you had advantage you did not have to use reckless attack at all.
What could be really fun is a whip (slow 10) + Slasher (slow 10) + Hamstring (slow 15). And the beserker damage boost still means you're dealing plenty of damage.