I can think of a lot of arbitrary things that I think are harder. Every limiting factor to the pros is by definition that hard.
Running a sub 4 mile. Running a sub 10 second 100.
Being quick enough to be an NFL CB.
Being 300 lbs and agile enough to be an NFL lineman.
Scoring in the paint in the NBA.
NFL QB reading a blitz and making the right call and making a great throw.
Carrying the ball 30 times in an NFL game and not getting hurt.
Hitting (as the server) a tennis service ace at the elite speeds.
Skating for more than 30 seconds without falling down.