Probably their only weaklink is the RB spot. They don't any big name for a RB spor and also CB/LB. If Vermaelen is playing as CB then CB is a problem area for them and if Vertonghen is playing as CB then they don't a very good LB.
Also they need De Bruyne to play in the midfield, Fellaini, Dembele and Witsel is too solid with no creativity at all. It's like watching Spurs midfield, too solid and lacking creativity. Fellaini and Witsel in the doube pivot with De Bruyne as a midfield trio, Hazard, Mirallas and Lukaku as attacking 3.
Imagine Belgium trailing and they just play Benteke, Lukaku, Fellaini, Kompany and thrown everyone in the box. Near impossible to defend set pieces.
Also Benteke/Lukaku, Dembele, Witsel, Fellaini, Kompany are all physically beasts. So much strength in the team.