Effects of PWM the source voltage for DC & BLDC motor?
The question mentions DC &amp; BLDC as if a single answer is expected for both. However there are two completely different situations.

In the case of a brushed DC motor, you could control the speed by PWM of the source voltage and use the H-bridge just for direction control. That would work, but there would be no electronic protection against excess current due to accelerating too quickly or overloading the motor. You could ramp the voltage up to control accelerating current and implement some kind of electronic shut-off for overcorrect, but you would end up wanting an active current limit. You could implement that with the source PWM, but that would amount to an armature voltage control with an overriding current loop. The end result would be a control system with more power components, a control scheme as complicated or more complicated than usual and poor performance.

The case of the brushless motor, you would be feeding PWM DC to a six-step inverter and starting with an open-loop, three-phase permanent-magnet synchronous motor control. You would have the same over-current protection problems as described tor the brushed motor and more difficulty in designing a remedy for the problem. In addition, the performance would poor. Here again, you have added complexity and subtracted performance.

Historical Note

The following block diagram shows how a PWM source voltage brushless motor controller (mostly for induction motors) was implemented about 50 years ago. About 45 years ago, that design was obsolete.