I have given this some thought, and have to conclude that it is potentially game-breaking.
Here is why: Elven Accuracy feat.
Elven Accuracy combined with Reckless Attack and Brutal Critical would be massively powerful. But this is forbidden because Elven Accuracy doesn't work with Strength based attacks. However, Elven Accuracy DOES work with Cha based attacks.
Even if your player is not an elf, you could have another player create an elven barbarian with a single level hexblade and demand to be allowed to use this exploit.
Elven accuracy is itself playtest material, a DM is perfectly in line not allowing it.
Edit : as pointed out, that's incorrect. It's in Xanathar's now. Below still applies though.
Heck, an oath of vengeance DEX paladin could abuse the feat as much or more than the build in question.
As for consistency: anytime I allow a build that deviates from the RAW it is with the provision "unless it causes problems..." This would be considered experimental and may have to be changed.
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