I have been asked in a couple of threads to give feedback on my recent trip to the USA since it is a rather popular destination.
I have to say, overall my Mondo card proved to be more useful than my RBS debit card. I STILL RECOMMEND YOU HAVE A BACKUP CARD.
I have started in NYC & Jersey and travelled my way through Dallas, Las Vegas and Florida. My Mondo card seems to be accepted everywhere I tried it (from diners and local pharmacies to clothes stores and dunkin donuts).
The only issue I had so far was with ATM withdrawals. If you attempt to take cash out in a random ATM in a wee store, odds are it will not work. The Mondo app was quick to let me know it was due to magnetic stipe withdrawals not being supported. I emailed support and I was advised that I should look for actual bank ATMs since they support Chip and Pin. I did a couple of withdrawals from Chase and City Bank and it worked perfectly. (Side note - ATMs in the US tend to charge roughly $3 for withdrawals). IF PROMPT TO CHOSE BETWEEN DEBIT AND CREDIT YOU SHOULD SELECT CREDIT. Debit/ current tends to fail (don’t ask me why).
If you’re just paying in a shop, feel free to just swipe your card. It should still work.
For the NYC Subway it will not let you use the automated kiosk to buy the metro card. This is because it will ask for the zip code in order to confirm your identity. The same happened with my RBS card though. If you go to the manual booth they will process your payment no problem (or you know, you could just use the kiosk and withdraw money from the ATM in advance).
I saved so much in foreign currency fees as well by just using my Mondo card. It is truly one of the best things that happened to me 
PS - any time I had questions about the mondo card and texted support through the app they always took less than 10 mins to reply which was great.
Any questions you have let me know and I will try to help.
@ross_o