Not true, you can't make sweeping statements like that.
How old is the BMW? Newer BMW's are using new clearcoat's, which cure over a 6 month period, so are initially very soft, and are never 'hard'.
E46's are generally 'harder' but never assume anything until you see, assess and test the paint on each individual car. Never go in 'heavy' assuming paint hardness. Always spend time testing combos out.