The effects of back-EMF to the output voltage from the DC-converters?
DC Motors are indeed bobins with polarized Magnets. 

If you apply DC voltage through pins of it, it will have no voltage drop because voltage drop is caused by AC currents.

This is the output of converters(Buck or boost does not matter):



SMPS converters(both buck and Boost or Buck-Boost) have switching Frequency, this way they regulates the input to the output. However, they produce AC + DC signal instead of DC, infact. (A pure DC is not exist in my opinion any way). DC level of the output may not cause to flyback effects so EMF, but ripples on the output will cause some voltage drop between pins of the Motor. Because you know, DC motor is basically an inductor.

When you drive a Motor like this below:



20 mA ripple(which is pretty acceptable) with 1MHz switching Frequency will cause more voltage drop. Because, DC motors have enormous inductance. So this representation seems right to me.