Rugby Champions has now been updated so that the AI v AI quick sims produce more realistic results. This means you can expect much tougher games in the playoffs in general, because poorer teams are less likely to qualify. It still remains possible for poorer teams to win, but just less likely. So in theory, the All Blacks could get knocked out in the quarter finals, the Springboks could lose to Japan, and England could fail to make it out of their pool. Of course none of those things would ever happen in real life ;)