Realistically - we're always talking probabilities here, because theoretically a transaction could always be undone using enough hash power - as long as the network (the complete hashing nodes, aka miners) are functioning as per the design, you're safe using 1 confirmation or even 0-conf transactions (as with Bitcoin Cash). But only as long as the network is running smoothly, with low fees, etc, so that you know there is a high probability that said transaction will not be reverted, thanks to the incentive structure explained in the design paper.