See the demerits for opting composition scheme is that –
1. Assess shall not hold interstate purchase stock on the day of exercising the composition schemes.
2. Assess is not allowed to issue Tax Invoice instead issue Bill of Supply.
3. You are not eligible to charge tax from your customers on the sales made to him.
4. You are not eligible to have input tax credit means to set off. As simple when a person is not authorized to charge tax then the same shall not be eligible to avail input tax credit.