Can we use a low side N-channel MOSFET in a DC-DC Converter for limiting inrush current, that is to say adding a NMOS just before the supply negative side and thereby controlling its slew rate so as to create a soft start? Are there any drawbacks to the same? I've seen many examples but mostly with high side PMOS (high Rds on) or NMOS (additional circuitry to drive in high side). I doubt there must be some reason.
This can definitely be done. However, like any low side control, you should only do this if the return of your load is isolated from the ground of the source.
In many electrical and electronic systems, exposed conductors (like shields or chassis) are at ground potential. Thus it's not safe to have two systems (your power supply and load, for example) with different "ground" potentials, because they could easily be shorted together. This, combined with the convention of positive supplies (i.e. systems where ground is the lowest potential) probably explain why it is easier to find high-side inrush control solutions.
You can find both ways of achieving this. You may also use a series resistor with input cap that you will bypass using a fet/thyristor/triac. It was mentioned your "ground" won't be exactly "ground" anymore but it's close enough that it usually won't matter.
If you google for inrush current limiting you will find many examples. Motorola AN1542 shows a simple method of high/low side limiting with mosfet using gate capacitance to do something useful.
Note that on AC/DC circuit the current limiting resistor is subject to a lot of power. It may be only for an instant but a regular 1W resistor would die horribly.
Using another mosfet just for soft-start is probably not economical, but would most likely be possible.
On Linear technology switching regulators / controllers, there is usually a soft start pin that you can hang a capacitor on. This limits the current that the IC will allow the circuit to draw from the supply, ramping it up slowly. This is the preferred way to do it, as a capacitor in mass production costs less than a penny, and mosfets can be in the dollar range.