Why the implementation of current sensing network (CSN) by sensing the switch current is simpler than that of sensing inductor current?
The switch current can be sensed by a small value current sense resistor between the source and ground.  Since it's ground referenced it's easy to use in the control system.  Also the temperature coefficient of the resistor is small, in contrast to using the RDSon of the FET or the ESR of the inductor.  In integrated solutions a FET current mirror can provide low-loss sensing.

Inductor current sensing requires a resistor in series with the inductor, or a network around the inductor to cancel the reactance and provide an estimate of the DC current.  Either way, it is not ground referenced and one end is switching rapidly.  The circuitry to sense and translate the current signal requires a differential amp and possibly a filter to reject switching spikes.

If you use a network to cancel the reactance to get the inductor current, the temperature coefficient of copper can be a problem, and sometimes requires compensation by a thermistor.

[With inductor current sensing we are looking for the voltage across the ESR of the inductor.  If we didn't use an external network to cancel the reactance we would have V=Ldi/dt+iESR.  What we want is just i*ESR.  This technique is fully explained  HERE ]

Bu sensing the true inductor current however, you can use average current mode control.  This avoids the subharmonic oscillation issue.  It does make cycle-by-cycle current limit harder to implement though.