I mean, a player could be Barbarian 2 / Rogue 1 and get this same benefit.
So what would be different about having a Rune Knight / Rogue 1 do it for 10 rounds, once per short rest versus a Barb 2 / Rogue 1 that could do it every round, for eternity, plus a mostly d12 hit die?
One's a level 4 character, the other one is a level 3 character.
I think it's people not actually playing the UA version of Rune Knight and simply looking at it on paper and complaining about it that made them change it and, yes, nerf it unnecessarily.