what is the disadvantage in using mosfet at input stage?
In this circuit the NMOS is used to get a high input impedance.

The gate of a MOS transistor behaves as a (small) capacitor, it does not require any DC current.

Note that here the Gate is DC-biased at 0 Volt. Some DC current must be flowing through that MOSFET so the source will not be at 0 V DC due to the source resistor dropping some voltage.

So \$V_{gs}\$ of this MOSFET will be negative in this circuit!

That means that the NMOS must be a depletion type (normally off) NMOS otherwise no current would flow and the amplifier would not work.

Although the schematic suggests that M1 is a MOSFET, a JFET (which are also normally) would also work.

In my opinion the MOSFET is not really needed.

You can also make a similar high-impedance input using BJTs, the circuit would have to be slightly different though. Many opamps have high impedance (about 1 M ohm) inputs using BJTs. One solution is to use a Darlington configuration.

But you asked about pros and cons:

Advantages of using MOS input:

high input impedance

simple circuit

Disadvantages:

Could be more noisy, MOSFETs have more noise than BJTs, a JFET could be a good compromise, they are less noisy than BJTs

Hard to find. It will be difficult to buy a depletion mode NMOS as they are very rarely used. JFETs are easier to find. BJTs are very easy to find.