Buck Converter vs plain PWM Power modulation
This will work and in fact is used in many circuits. A brush DC motor in a RC toy is feed by PWM and speed of the motor is a function of a duty cycle. A light dimmer works on the same principle, even though the waveform is not rectangular.

Don't expect accuracy and also understand that time is often a factor. An electrical motor has max.torque (and power draw) when not rotating. The resistance of a cold heater is quite a bit smaller than that of a hot one. If you set some fixed duty cycle and never change it your heater will be heating up much slower.