This Question is somewhat subjective. Football is a low scoring sport. This means that one point (goal) is so much more impactful than one point (Touchdown, Goal, basket) in other sports like in American Football, Handball or Basketball. Due to this, a team with weaker players is able to win if they have one good play while being good on the defense.
It's also worth mentioning that certain teams excel at different things. For example, Barcelona's Tiki-Taka is extremely good at circulating the ball at a high tempo, keeping posession and allowing for the ball to safely go up the field. This Works because Barcelona has fast players that are good at passing the ball. Should they meet, Mourinho's Internazionale (which happened), they'd be in serious trouble against his team's deep lines with pin point long balls. The outcome was a clear two-nill that put Inter in the Finals of the Champion's League.
There is however something else to consider. In the World Cup, each group has 4 teams, and only 2 go on to the Knockout Stage. If we consider the Group B of 2014's World Cup:
You had Chile, Spain, Australia and the Netherlands in the same group. By Quality, Spain and Netherlands would go over. However, Chile beat Spain, and ended up in 2nd place. Due to Spain, the stronger team being out, it means that, potentially, in the finals they wouldn't have to face Spain, because Spain was already knocked out. You could argue that if you beat Spain it's because you're stronger than Spain, but that's not always true. In fact, Spain(Didn't make past the group stage), Brazil (4th place) and Germany (winner) were the favorites going into the WC.
In Sum, the effect that winning the group of death has on a team is, other than raising morale, simply not having to face the colossus you beat already, because, in theory, the weaker teams that were in other groups will now be playing against you (since the stronger teams were already out of the competetion due to you winning the group of death)