The biggest pain I have found has been local transport. Buses, trams and metros. Having to purchase tickets from kiosks, from kerbside TVMs, or worse having to download a specific app just to pay a bus fare in a small town in Romania -
Indeed.
I had fun in Bucharest this summer.
There was a sign on the surface local transport ticket vending machine at Gara Nord saying that it wouldn't sell me a ticket but giving a list of alternatives.
SMS payment? Not with a foreign phone, it seems.
Download the app? Again, not on the British app store (and yes, there may well be a way round it but not really what you want to deal with when you're just trying to buy a ticket to get on the next trolleybus).
Contactless card - if the bus has a reader, but that isn't guaranteed.
I forget what else but none of them worked.
Fortunately there was a manned ticket kiosk but I don't think there are many and I have no idea what I could have done without it other than faredodging, walking, or getting a taxi.
Going back a few years, finding a shop that would actually sell me a tram ticket in Milan was something of a challenge.