Winning is great, but if you are winning a lot of games by last-second field goals and collapses by opposing teams, it should make you worried that it might nr be sustainable, especially in the playoffs, when the competition gets tougher.
Nobody is trying to take away the Vikings victories, or say that the team is terrible. But the let's just do a little thought experiment. Given the exact same win-loss record, would you or would you not feel more confident about their chances to advance far or win the Superbowl if their average margin of victory were, for example, 17 points? Be honest.
In the event that you WOULD feel more confident with an average margin of victory of 17 points, you have just proved that the Vikings' critics do, in fact, have a point, even if they are perhaps over-selling it right now.