I've been using Revolut to transfer money between countries because of its near-perfect exchange rate on weekdays, but I fear the day my account would suddenly be locked for weeks because I hit some secret anti-money laundering limit, or because I'm suddenly seen as a customer who is just making them lose money.

I had that happen recently. Fortunately I was not in a rush. I had to complain on social media after the situation hadn't been resolved in a couple of weeks, then magically I got some proper support.