I don't get it???

I have used the spray method on 4 different 90s williams machines and it works VERY WELL...

I spray heaps of the liquid (can upside down) on and the mylar litterally lifts off (makes a crackle sound) and requires very LITTLE force and has never lifted ANY paint so far. In fact if force is required I stop and use much more freeze spray on that spot.. As I said I recently did a full playfield (dr who) that the mylar was bubbling on.. Of course if the mylar is well stuck and doesn't look bad I would probably just leave it...

I have heard though it depends on the era of playfield, on old playfields it is far more dangerous than on 90s playfields..

Heat can be very dangerous too, I have heard of plenty of people using too much heat with heat guns and damaging the playfield, the best method with heat I heard is leave the playfield in the sun on a hot summers day and then peal off carefully...