The only nation to use floating head pistons was the U.S.

Floating head pistons were banned the year after the U.S. won altitude and scale altitude at the World Championships in Ukraine.

In all fairness, though, one of the objections to floating head pistons is that our piston tubes would fly 20 - 30 feet in the air and then come dropping down on the heads and rockets of our neighboring competitors. Unlike U.S. competiton, International events have each country launching from their own narrow lane. Having piston tubes raining down on your rocket sitting on the pad was pretty annoying. Don't recall that we ever knocked anyone's rocket down, or hit someone in the eye, but they did have a point.

Floating head piston are still allowed in NAR competiton and are definitely way easier to set up than fixed heads.