The position of isotopes is also considered a demerit of Mendeleev’s periodic table.

The above statement is True.

Isotopes of an element have different atomic masses but the same atomic number. Since the periodic table has been framed on the basis of increasing atomic masses of the elements, different positions should have been allotted to all the isotopes of a particular element. So 'positions of isotopes' is also considered a demerit of Mendeleev's periodic table.