That's a very interesting question. Is realism in games a thing? Well, obviously for UrW it is, but for others no. In my experience the UrW is, what I think, one of the more realistic games about survival, but is it really realistic? well, no; that's somewhat impossible for a videogame. There are many things to be considered like balancing, and obviously, fun (that delicate line between tedious realistic work in-game and reward for the user).
Therefore, does BAC unbalance the realism of UrW? I would say no, on the contrary it brings more realism.
Maybe you can find Finnish realism discrepancies (like the birch-bark punt Brygun mentioned), but in general it just adds historical iron age heavy-inspired-realism processes. Balanced and, as the same UrW authors said, with some artistic liberties for the purpose of make an entertaining game, not an documentary about Iron Age Finland (although UrW can be considered a similar thing, is very educational
).
I think that if the game goes 100% realistic wouldn't be fun. Life was and is hard, and the struggles for ancient people were a thing. But there is a well said principle you can find in Aristotle's Poetics: The spectator will be satisfied if the story presented to him respects the own rules of the world representation delivered. For example, you maybe don't know what a giraffe is, you may never seen one, but if someone came and explain it's properties to you, respecting the rules of physics and biology that governs 'your' world (this very world), you would understand what a giraffe is and be satisfied seeing a characterization of that. A similar thing applies for UrW and BAC, both gives you elements of a world that you can say "oh, yeah, sure it was a thing in those times".
And in addition, in my gameplay experience, I would say it is balanced. If you read the code it has limitations for keeping things difficult as those things were in those times. Mining bog ore was a thing, but the process would surely be a slow one. The same goes for 'carrying tree trunks' and 'building a house'. Man that was
a thing. You don't want to do it in actual life in the same way you do it in UrW, is really tedious and hard work. That's why lots of cultures had rituals for many things, those were very important accomplishments.