The least protected part is also the most dangerous part if it is damaged or looses integratiy.
The entire Tailboom on every helicopter regardless of attack helicopter or civilian purposed have unarmored tails (tail boom). They are simply aluminium sheets, if explosion rips even a 30% hole in it, the integrity will be gone and the stress the tailrotor and torque of both main rotor aswell tailrotor shaft creates will rip it apart and helicopter aswell crew are lost.
Rotor blades are much harder to destroy. Missiles have very low payload to deliver, if it detonates beneath the rotor disc in between two rotor blades, most of the explosion and energy will not even hit the blades but just create a vibration, such vibrations have been spotted on Mi-8's in Afghanistan, they are very frightening to the pilots but often do not cause destruction of rotor blades. Those blades they used back then were not even armored or special durable rotor blades like are used today.