Both the nails and hair are composed of the protein keratin and as they are both parts of body so rationally you should not cut nails either.
OK. For practical reasons the Gurus prescribed nail cutting but just because they said you cannot cut hair does not make it a rational decision; there is no rational reason not to cut hair so it is a religious obligation.
As for armpits etc, it is not a religious obligation to cut or not, just like some keep beards and others not. Hygienically, is it better with hair or smoothie? Make your own mind up and do what you prefer.