The Nuki Smart Lock works with a force of 2Nm - this is about the force an adult has turning the key with one hand. So in principle, stiff doors are no problem for the Nuki Smart Lock. However, the stiffness of the door is related to the battery consumption. The stiffer the door, the more current has to be used to power the motor of the Nuki Smart Lock. This reduces the battery life.
To keep battery consumption as low as possible, always check if the key can be turned easily and without great effort when the door is closed. If more force is required to turn the key when the door is closed, it is best to contact a professional to adjust your door for you.