Oddly enough - its only really bally and Stern games that ive had bad results with.

Same here. i own three Stern machines and one Bally, they are definitely the worst ones for lifting paint on the playfields when removing mylar.

The paint/ink preparation, especially on the old Stern playfields for the artwork must've been pretty bad i reckon. Also they're bad for lifting plywood up with the mylar too... this happened on my Dracula.

Gottlieb playfields seem much better for paint adhesion, i havent had many probs with them removing mylar.