I definitely recall there being no fall out or or problems when ruby did it in I think 1.9. Semantically, I can't think of how there would be. Since previously order was unpredictable and not guaranteed, it could be anything at all; always using insertion order is, after all, one possible value of "anything at all".

Only possible downside might be performance (or memory). I recall the ruby maintainers ensuring they had an implementation with very little if any performance/memory implication.