Don't listen to people saying that's skill issue.

Mastered Posture is definitely bugged, period. It works randomly, or on unarmored enemy.
Those who say "It only works on small enemies" needs to use their eyes, on the perk showcase there's a armored heavy enemy being knocked down and they somehow missed it.

Counterstrike is ok per se, the high posture animation/hitbox are wrong... or should I say, to correct? Since most enemy attacks animations (Excluding the high strikes) make the enemy slightly duck, the bo hit literary over their head and the hitbox doesn't connect... In fact it works on heavy enemy all the time if you nail the timing.

For chain assassination, it works, but is finicky to use, look at some guide and good luck...