What are the demerits of joining the dual degree program at IIITM Gwalior?

The only demerit of joining the dual degree program at IIITM Gwalior is the skills you will learn in the five-year program can be quite easily gathered in four years’ time with little effort. Based on your interest(s) you can always be a B.Tech graduate and/or an M.Tech postgraduate.
The only disadvantage is the extra one year that you will have to complete for completion of the program. The decision is up to you about what course duration you want to go for. At the end of the day, it all comes down to what benefits you are getting be it a four-year or a five-year program for the same degree. As long as you are getting your money’s worth, it should be good.
The dual degree is eligible for GATE, UPSC, Ph.D., MBA, and everywhere there is eligibility for B.Tech and M.Tech.