In terms of Europe, our Mastercards are hit-or-miss due to many merchants requiring 3DS enabled cards (which these are not, due to not having a name or address associated with them). Our prepaid cards tend to work well in Croatia and Germany, and often times at larger merchants (like McDonald’s) for small value transactions (< 30 EUR). However, we've had intermittent issues reported in other countries in the EU.
Note that this does not apply to our Visa cards, as our Visa cards are only able to be claimed and used within the US.
The best strategy is to purchase a relatively low value card for testing purposes (eg, less than $10). If it works with the retailer you're intending to use it with, then you can scale up your purchases from there.