I'm a firm believer that temperature change (varying expansion rates in the glass vs the ink) is the greatest risk. Bought a Rolling Stones (Bally) pinball, and on the way home the back glass delaminated significantly. Would never try mylar.... we've all seen it bubble on playfield. Back to the Rolling Stones. If you look at how the back glass is illuminated on the machine, my delamination occurred where the greatest concentration of bulbs were... Heat. So I now back light all of my games with LED's.
One other comment...Somewhere in this thread I saw where someone was having trouble getting a good wet coat on. I only apply one heavy wet coat and have gotten great results... However, the ambient temperature has a lot to do with how Triple Thick sets up. The warmer it is the faster it sets up which makes it difficult to get that "wet" look. I solved this problem by using two cans of Triple thick simultaneously... One in each hand. Works great. Also have had great luck painting outdoors in the 40's. Just have the glass nice and warm in the house and also be sure the Triple Thick is warm as well. When I'm ready, I take the back glass outside, shoot it immediately, wait about 5-10 minutes, then take it back in the house to cure.
They won't last forever, but I think Triple Thick is one of the best options.