what is the disadvantages of overhead watertank in northeast area?

Overhead tank should not be in northeast of the house. The tank will amke this direction heavy which is a big defect of Vastu.  You can put water tank in northwest of the house, but the tank should be small in size.

                                     As per vastu recommendation you should place water tank in the west or southwest direction of the house b'coz these directions are negative zones of the house. Due to water it become heavy and balanced the energies. 

Regards:

Nikita.