As a Slovene/Slovenian, I can confirm that credit card coverage is quite good. You may find small local stores that only deal with cash, perhaps more frequently in remote areas, but that just means you got lucky.
Even if you find yourself in need of cash, it shouldn't be a problem finding an ATM machine. You can find them here. The page lists location, operational state of the ATM as well as acceptance of Visa/MasterCard/Diners Club cards.
MasterCard has an online database that enables searching for merchants that support their contactless payment implementation. Select
Slovenia and enter one of the postal zip codes found here. Note that not all merchants support contactless payments, but may still support MasterCard, so you should take that into account when checking the results, but you could get a general idea about MasterCard coverage.
Contactless has been on the rise here for the last two or so years, since our banks decided it is the next big thing and started replacing our debit cards with ones that support the technology.