http://paraglidingforum.com/viewtopic.php?p=p263535

the extra reinforcement in a 2-line wing can prevent the wing from
recovering from an otherwise minor event

the structure of a 2-line wing makes it more prone to strong surges when
suddenly loaded (by a large asymmetric collapse)

high aspect ratios are inherently more prone to dangerous behavior a pilot's
competition mindset can subject him/her to greater risk

a 2-line wing that enters a frontal collapse is slow to
recover from the frontal, making a horseshoe more likely

a much shorter reinforcement in the leading edge, combined with

a larger gap between the A-reinforcement and the B-reinforcement