Very nice. 3x 1d6+Strength+Dex+Rage damage is pretty respectable.
Anything that gets Barbarians away from great weapons is fine by me so I would allow it.
I have not like the Barbarian class since 2E though there it was a tough outlander non magical Ranger type PC not a rager (which was 1 option).
The way I look at it is the feat rewards you for trying something different kind of like how Monks rock with spears but spears overall are a bit meh.
The feats broken with really high rolled stats but so are the usual suspects. My level 6 fighter feat is weapon specialisation also 3pp from the same document.
WS
+1 to an ability of your choice
+1W damage on a crit
reroll 1's a'la halfling (Halflings can now reroll 2's).
So at level 6 with rolled stats I'll have 20 dex and 16 strength, 14 con. The DM kind of likes it as I am playing a Champion and he likes fighter types but also its original. Its not hard to rock on up with a EB spamming Warlock or a -5/+1- build and we;'re sick of seeing them. Normally you would not pick a halfling or a champion as a great combo or option for top tier melee (more Paladin, Battlemaster or not Halfling).
The link in my OP has the DMGuild link. It also has an option for Sorcerers that lets you substitute your energy (as a bonus action) so non Fire Sorcerers also suck less. It is power creep over the PHB true but its more sideways vs something like Xanathars which is pure power creep (Hexblade for example).
Both DMs when we evaluate 3pp kind of look at it as "Is this feat worse than -5/+10" and if the answer is yes its probably fine. Even turning a fireball into a frostball is less abusable than some of the PHB options.