Are parasitic plants harmful for the host plants?

Plants are affected and, in extreme circumstances killed, by the same range of parasites and pathogens that cause diseases and disorders in animals and man, though the number of parasites in the various groups is probably different. (It's difficult to be certain as the presence of some pathogens or parasites is difficult to detect without sophisticated diagnosis and there are a great many plant species.)

The commonest pathogens are fungi (mainly microfungi) and viruses, followed by bacteria and phytoplasmata (primitive bacteria with small cells and lacking a cell wall).

Depending on your definition of parasite, many arthropods (insects, mites) and nematodes feed on and in plants causing various degrees of damage and, incidentally, often acting as vectors or facilitators for the ingress of the microorganisms listed above.