Gordon's said when I read up on it that uls has better lateral stability. Correct me if wrong, but I took it to mean on recurves to help tip twist.
Also supposed to help keep the bolt holes from splitting out on TDs but I think most prob there is overtightening bolts or not getting enough finish in hole and wedge swelling and splitting the glass....
The clearest glass I think is .050 ul , from what I sorted off when selling it. YMMV