Simply put, it's hard to tell because there are differences between 5e and BG3, so pointing to 5e isn't necessarily authoritative.
Simply put, if something is inbalanced, they should probably fix it.
Possibly, although I'm not convinced balance is possible without resorting to everyone playing the same character.
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That said, I think it has to be demonstrated that the extra attack is somehow unbalanced to such a degree that it needs to fixed. Why is that? I'd rather not accept it as a foregone conclusion because "that's not the way it is in 5e so I don't like it."
Let's say you go five levels in Warlock and the rest in Valor Bard, as that's the topic of this thread. Great, you get three attacks. You also give up higher level spells. And you give up invocations as a Warlock. On top of that, you give up a feat. In fact, you give up two feats when compared to going full Fighter to get the same three attacks.
It's all give and take.
