I've been in the Army, and no, most people dont fail. Most people climb them reliably every single time.
You grab the rope. Lift your own bodyweight off the ground. Lock the rope between your feet in an S shape (where you can literally hang all day long, basically standing on the rope) and repeat.
A PC can lift (Str x 30) lbs off the ground. A Strength 10 Commoner PC can lift 300lbs off the ground. Unless he's morbidly obese, (or has a Strength of 5 or less) he can lift his own body weight off the ground, loop a rope over a foot, and repeat, reliably, and with next to zero chance of failure.
If you cant do it, it's because you have custard arms or you're obese (Your Strength score is less than (weight in lbs/30).