The problem with measuring the current directly with the MCU is that the resolution of the ADC isn’t enough if you want any kind of low modes. A 50mV shunt (e.g. 10mR for 5A) gives you 200uV at 1/250 (8bit dimming) or 50uV at 1/1000 (10bit dimming), whereas a 10 bit ADC with 0.55V VREF (in the attiny 1 series for example) only goes down to 550uV. Increasing Rsense would lead to efficiency losses and wouldn’t be sufficient on its own, but if you’re fine with no low/low mid modes then it could works.
We talked about this in the 1 series thread , about the Unicorn and Gekko, they use special techniques to increase the ADC resolution (delta sigma modulation) to be able to read the Vsense at low modes.
After that the FET can be PWM or linear, or it can be used for a buck or boost converter too.