The Rollbar behind your head should protect you as the windshield header is not strong enough - with glass or plexi. It can collapse with either material. I've seen several cars that have rolled at the track and everyone walked away...your results may vary.
Please don't ever use plexiglass for anything other than a screen door. Polycarbonate is the way to go. Name brands are Lexan, Makrolon and a few others. They are virtually indestructible and can even be bent with pressure(brake) or heat. plexiglass will shatter and end up in your face if a rock hits it.
Now you can even get hardcoated poly that will withstand wiping and cleaning.
I've definitely heard of the dangers of Lexan windshields popping off in a collision and decapitating, or at least injuring passengers, but have never seen or read about actual incidents (granted I've never looked for this info). Do it at your own risk I suppose!
I am not concerned of it popping off. Nor andy structural significance.
Glass disintegrates, plexi/plastic/vynil/nylon any composite/polymer linear or crosslink are strong
I am concerned with the screen lowering toward face in a rollover. Where glass would fall apart, plexi would remain rigid
Any documented cases of plexi hurting its occupants. I suspect it would catch (scrape) on the ground before protruding onto ones head