I’m gonna say relaxing.
The thread title asks for “hardest technique “. But the op asks for “hardest thing” so we see responses to both.
I have found that every technique I try is as successful as practice I put in. But for almost every technique the main challenge to me is reaching a point of relaxed accuracy with whatever I’m trying.
You are doing motion alien to what your body is used to and when things get stressful the tendency is to tense up and to muscle through. That physical tendency is anathema to clean musical execution.
Fretting, plucking, slapping, tapping, rapid steady 16ths any technique is well served by a relaxed light touch. That lightness needs explicit attention during my practice
I think this is part of what Anthony Wellington refers to as “getting out of the way of my instrument “