It's not instant. According to https://www.crypto51.app/ the cost to 51% attack ethereum for 1 hour is $92k. This means it's $2.2 million for one day or 24 hours. Let's say you want to transfer $10 million via ethereum. Then the party who gives you the eth can perform a double spending attack, only pretending to give you the $10 million while getting something else from you in exchange as part of the original deal that takes one day to be transfferred to them. As long as that something else is worth more than $2.2 million, that other party has made a return. The only good remedy is waiting for the double spending cost to exceed the contract volume, but it means that the currency isn't instant any more, or at least is instant for small sums of money only.