I completely agree that they lack transparency, especially on the exchange rate applied. There are things you can do to minimize the risk: exchange money on a weekday in app into the currency you want to spend on your card, and check the rate on xe.com before making the exchange. Get another card for cash withdrawals abroad at the MasterCard rate if your main currency is GBP, especially for countries with supposedly “illiquid” currencies (according to Revolut): Loot or Monzo, I believe, offer the MasterCard rate, which is better than Revolut’s for illiquid currencies and sometimes also for liquid ones. And don’t think of Revolut as a bank: it’s not, it’s a prepay card, and should be treated as such until they stop the silliness with arbitrarily capping pre-applied topup limits, etc.
My son, for example, had a £25,000 top-up limit, confirmed in the app. But the moment he added a significant sum to do a foreign-exchange transaction, Revolut immediately blocked the amount and demanded proof of the origin of the funds. After he provided proof, they limited his account to £7,000 total. So, why pretend the top-up limit is £25,000 if it actually isn’t?