It is extremely harmful when people like them go around non-tankie left wing subs and eventually gain control through nefarious means and eventually transition it into yet another tankie propaganda sub. Or when they moderate a supposedly general left wing sub but any criticism of authoritarian communism gets you muted and called a liberal, eventually allowing their propaganda to seep through to regular socialists as any criticism is shot down. /r/GreenAndPleasant is guilty of the latter.

It's not really the politics but the way power hungry people get.