The NRF51 on micro:bit doesn't have a PWM, so indeed it's emulated. The NRF52 on micro:bit v2 has PWM, so you should see no jitter. RP2040 can also generate PWM

The NRF51 on micro:bit doesn't have a PWM, so indeed it's emulated. The NRF52 on micro:bit v2 has PWM, so you should see no jitter. RP2040 can also generate PWM signals in multiple ways. BTW, the problem with servo is not about "flickering not fast enough" but about length of the pulse. Servos require ~0.5-1.5ms pulse every 20ms, where the length of the short pulse maps ~linearly to angles of the servo. If you miss the timing by say 50us you get jitter.