Malthus Theory weaknesses are that his theory was untrue and what is real is that when the number of people continues to expand, the needed resources will expand also and any of them would not exceed each other and there is still an equilibrium.
On the other hand, Demographic Transition theory weaknesses include that the original model needs modification to add an unexpected Stage 5 evidently observed in Eastern Europe. The theory did not put into consideration the relocation of human beings which is the major factor to understand population as a inputs system, change of population and productions.