I think the brutality has to be adjusted by the context. The cartels are among cartels. They are constantly tempted by the prospect of wiping out their rivals and constantly buoyed in their brutality by retribution. That wouldn't happen where there is law, order and prison sentences.
The yakuza don't have to be brutal. In fact, if someone is brutal in a context where they don't need to be, that guy is not an asset- he's a liability. There's a reason why the mafia had rules, so nobody just kills anyone without permission.
Successful organized crime eventually joins the real upper class. The Italian mafiosi own too much real estate to take risk with scary illegal stuff. From one perspective, that means they've won.
So yeah, the Yakuza weren't hardcore and certainly aren't now, but that fits their needs.
How would the Yakuza handle the situation if I forced all of them and a few million lower class japanese into the middle of mexico, permanently? I think a short period of protection for sake of their adaptation is fair, maybe two months to learn the basics of the language.
I think we all know there would at minimum be a new cartel. Maybe there would be several fewer.
If it were the old yakuza I wouldn't be surprised if there were no cartels at the end, for the same reason that I would not expect there to be any cartels in Mexico if it were an imperial possession of a japanese empire. The nipponese are capable of violence, and they are capable of cruelty like we saw in WWII. If they are primed for it, they are certainly capable.
Side note, humans vs chimpanzees? 1v1 and 10kv10k matches scored separately.