I think 12 unbalances the prehistoric era pretty badly and will throw the AI for a loop. Main reason is that it's almost certainly a viable (and indeed probably optimal) strategy to not bother defending your city at all apart from with that auto-unit, which means that the AI will waste its early units on pointless defense instead of exploration, giving the human yet another advantage. If it's going to have out-of-era strength it'll need new AI writing to handle it because it modifies the optimal strategy too much (wars are essentially pointless too, but the AI won't know this, etc.). IMO we were just trying to solve AIs dying off to Neaderthal or attacks by a few stone throwers. A strength 3 or 4 unit should suffice for that purpose.