Edit: I have spoken with support. Revolut is unreliable in countries where no prepaid cards are emitted! As the card is a Mastercard prepaid, and prepaid cards are not a product offered everywhere, then transactions can (and as explained below, will) fail.
This should be clearly stated somewhere (the FAQ?).
Sadly I have to report that 50% of my transactions have been rejected in Chile. There is an uncatched exception in the payment machines. There seems to be a problem with how the card is handled at certain merchants, maybe?
The error is:
"WALK: POSXML error.
WALK: Invalid parameter size."
It seems to be a problem with some credit cards from the UK, see here. There is no issue with my bank’s Mastercard.
So beware that the card can become a a pain to use.