Ian Cant wrote
And while ultralight hang-gliders may have a little
less liability damage potential than a Nimbus

A lot less. It's much slower and much lighter, and probably is
capable of delivering no more than a quarter of the energy on impact
that a Nimbus can deliver.

they are flown by unlicensed pilots

Not true. Those pilots are licensed by USHGA. Sure, the
instructional program is not under FAA control. However, my
experience as an instructor both in an FAA-controlled environment
(gliders and airplanes) and in a non-FAA-controlled environment where
a sport association issues licenses and has a liability insurance
program (parachutes) leads me to believe that FAA involvement in the
training program does not add any safety or proficiency value - only
increased cost and bureaucracy.

under loosely controlled conditions.

Are they any less controlled than the conditions at a privately owned
grass gliderport? My (admittedly few) lessons with a USHGA instructor
suggest otherwise.

It seems that the two risks might be comparable.

It doesn't seem that way to me at all. Clearly the gliders involved
are not capable of causing near as much damage, and the proficiency of
the pilots is probably about the same.

Michael