It is supposed to be very weak in the middle, hence like you said, likely to concede a lot of goals, especially if attacked by opposing teams through the middle. But, I have noticed it to be strong in the middle in one match where I got a 1-1 draw against a weaker team. I have also noticed other teams using this formation getting good results. This looks anomalous. Something seems to have changed, that is this formation has been over-powered.