I see this often cited, but in reality it's a farce. "UPI is international" the staunch defender says, so I rebut "Yes, in one place at the Eiffel Tower... everywhere else? The French have no idea what UPI is, and your bank will charge you stupid FX fees for card payments".
Meanwhile if I'm in India, people look at me weirdly for paying with UPI yet most won't take card payments outside of tourist areas or it will get declined because foreign cards are blocked - and if you try to pay cash suddenly nobody has any change, will refuse to take the 20 rupee note they gave you yesterday, or have concerns about whether the notes you literally just withdrew from an ATM are legitimate - meaning you can end up with notes that are defacto unspendable despite being perfectly legal tender and in acceptable condition.
And as a tourist... you want UPI? There are a few ways but they're byzantine, apps locked to the Indian App Store (for tourists?), in-person KYC upon landing, very low first-payment limit, topup/signup and idle fees that push the net fee % easily into the 5-10% range.
Lets look at a perfect example... you pre-KYC on an app ahead of your trip on the one app that allows remote KYC, but you can't load money onto - first you must provide your visa, but the eVisa doesn't count they want the actual visa stamped in your passport. You land, immediately after customs you submit the picture of your visa stamp and wait an indeterminate amount of time, it could be 8 hours, or 24 or 48 or it could get rejected and you could be required to do in-person KYC (either you go to them, or they come to you within a ~5hr window... but only in the major cities).
So day 1 it's impossible to use UPI. It gets approved on day 2, you take a taxi somewhere maybe a nice restaurant, your UPI is now loaded with INR and you try to pay the driver... Your driver has a personal UPI account, you can't pay him! You only have cash... Large denomination INR notes because that's what the ATM provides, he doesn't take card and refused to admit he has change. You eat the already inflated cost and swear to only use app-based services (assuming the Taxi Mafia hasn't had them banned in your city).
You get to the resto and enjoy a meal with your friends, it's a nice place and somewhat expensive, you come to pay, the bill is reasonable and you think "I will pay with UPI", you try paying, it's a business account so should be OK! NO.... You have exceeded your first-day limit! Waiter tells you there is no card machine, and they have no change for cash...
Eventually you leave India, there's a non-trivial amount left in your tourist UPI account, you look for somewhere to withdraw it back to your card - no physical counters open at the airport, you request a withdrawal via the app... it never comes, the next month you get hit with a 500 INR inactivity fee, your visa expires and the app shuts down, next month 500 INR inactivity fee - can't make support requests through the app any more because your visa is no longer valid... Your balance slowly goes to 0 because you didn't think to spend every last rupee on your way out so it gets eaten by the system.
I say a small sub-1% fee on UPI is fine, it's great infrastructure when it works, but more needs to be done with global UPI integration. I have QR enabled payments available across maybe 10 different countries and India sticks out as being the one that's consistently an absolute pain and actively works against you.