The biggest problem with uPVC doors is that they are inherently weak mainly due to them using a 22mm to 28mm thick board as the main part of the door. This board is just foam with a thin plastic skin which is not very strong and also not a great insulator.
Composite doors on the other hand use a 44mm thick slab. The solid core door is made from sixteen layers of laminated veneered timber called Kerto. This ensures the grain is not running in one direction. The grain direction can sometimes be a weak point of timber and can lead to warping, or splitting. By using a specially orientated layup of sixteen veneers, the grain is running in multiple directions ensuring the Kerto core has significantly increased strength, rigidity, and vastly improved security. Bonded to this are two robust coloured PVC-u skins that offer a realistic wood grain effect or smooth finish in a range of colours.