Kichwas
Half-breed, still living despite WotC racism
Watch the YouTuber 'd4' - He's been transitioning from 5E slowly over to making more and more videos for PF2E.I can't say I've seen particularly visible balance-breaking from Free Archetype. In practice, what mostly happens is some additional options, so there can be some shopping for abilities that are useful in a given situation, but as someone said, it's horizontal power not vertical.
He's still only using the core book, but he's also "busted" the game with some builds.
Consider a Dwarven Fighter that Takes Barbarian and uses a Maul. You can use all those fighter abilities, while using rage to pump them even higher, and spam knockdowns and crits. And because it's done with Free Archetype - you still have just as many feats left over for your Fighter stuff as the core rules intended.
This combo makes a fighter even more obscene.
You can repeat this with a lot of class combos. Combine two classes and things can get out of hand. It's not a problem if you do not allow free archetype to be used for multi-class - but this then means no one can do multi-class in a free archetype game until around level 6 or so (when they've managed to get the dedication + 2 more feats of that dedication, possible by level 6 if you use your normal class feats to do it. MIGHT be possible by level 4 but I haven't found that combo yet).