As summed up by a reviewer of Joel Dimsdale's recent work Dark Persuasion: A History of Brainwashing from Pavlov to the Social Media
"The term has always had more political explanatory power than actual psychological basis".
"Brainwashing" is most usually used like it was some sort of swear-word, rather than being an accurate description of anything. All too often the accusation of "brainwashing" forms just another type of ad hominem attack - and frequently used to bully the other person during discussions about (typically) religious or political matters.
That was certainly the case in the household in which I grew up, anyway!
(And as was noted in another recent thread on this discussion board, ad hominem attacks only begin when the other side "has nowhere left to hide").