What is wrong with this simple SMPS?
The main problem with this schematic is that there will be a moment during switching when both MOSFETS will be conducting current and then will short the power source. As a rule, this moment is short and will not burn the MOSFETs, but the efficiency will be affected and there will be high surges in the power source. 

Replace the lower MOSFET with Schottky diode in reverse. 

Yes, using a MOSFET can increase the efficiency, but then the schematic needs special driver that to make dead time between switching transistors ON.