Well, I started experiencing this last week.
about 10 days ago, the “interbank” rate was around 39,04 THB per EUR
and Revolut exchange was 39,02.
This is still within the acceptable margin.
But yesterday, the situation was this;
XE currency rate: 38,75 THB/EUR
Thailand local exchange/ major bank office: 38,50 to 38,70 THB/EUR
Visa/Mastercard international official exchange rate: 38,44 THB/EUR
Revolut rate: 38,13 THB/EUR
The difference is amazing. And for me, this is a big disadvantage, as 80% of my travels (outside of EURO-zone) is Thailand.
I would expect Revolut to be able to offer at least the same rate as Mastercard if not better, but as you can see above, the difference is quite big.