The word that best describes these pictures is HARD

After days of racking my brain around possible answers, I've come to the conclusion that all the images shown above describe something that is hard.

IMAGE 1: Nuts are HARD to crack.
IMAGE 2: Rocks are known to be HARD.
IMAGE 3: This particular exercise is hard to many people and can only be achieved after much practice and flexibility. So generally, it is a HARD pose.
IMAGE 4: Mountain climbing is difficult, HARD and can also only be mad less difficult with practice. Generally too, it is a HARD activity and not everyone fancy it because of how difficult and dangerous it can be.

Thank you @manuelhooks for another week of brain exercise.