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...
I think the Freeze method works best on clear coated pf's with out to much drama
I don't know what method works with older games that pre date Diamond/ Stealth coating
the only problem I ever had removing mylar was with heat
and I think I forced it before i heated it up properly
you might get better advice if we knew the name of the game