poor positions in the area, and not tracking runners, but ultimately man mark the full backs, you stop the goal at the main source which is the cross, still cant get my head around how much space players are giving in wide areas.
years ago it used to be a whip low cross attacked at the front post, Fletcher was excellent for us doing this, Giroud as well more recently
Take City for example.
If Bernardo holds the width, De Bruyne in the half-space will jog inside the full-back and Bernardo will play the ball round the outside of the full-back for a cut back.
If Bernado comes inside, Walker will be in Bernardo's wide position and Bernardo will occupy the half-space, jog inside the full-back and the same situation as the above with occur.
They've created a system entirely based around
moving the opposition defence in order to create numerical superiority on the wings. Always 2v1. Man-marking is pointless because the person doing the underlapping and the person holding the width alternate all the time, so having assigned men to follow will just destroy any organisation.
Three CB's theoretically would have an easier time negating the numerical superiority on the wings and be in a place to clog the channels, but as always, there's always a compromise elsewhere on the field to make this happen. At the end of the day you can't prevent numerical superiority as a factor in football. It's impossible. You just have to decide which areas of the field to allow it in and which areas to not.