"It's easy, just use contactless" is something I often hear, but it's not always easy to know how they expect you to use contactless in that city.
In London, you tap on to a bus, but not off. In Amsterdam, you must tap also off a bus/ tram, because the fares are zonal - and if you don't tap off you'll get max fared. In Brno, you don't have to tap off, but it might get you a cheaper fare if you've made a short journey (like the German Kurzstrecke). In Prague, you choose a 30 or 90 minute ticket from the machine on the tram, then tap your card.
On a metro, it's easy when there are gates. Tap on the way in. To exit, there may be card readers (London) or no readers, just push the turnstile (Rome). Or certain gates you must find (Brussels). But when there are no barriers at all (Brescia) I was left trying to guess whether I was supposed to tap out for the correct fare, or whether it was single fare, and tapping again would charge me twice.
And don't even start me on whether you can share a contactless card with others in your party (OK in New York, for example).
Good luck guessing what to do for a second bus/ tram. Was your ticket a timed ticket so you don't need to tap again? Or a timed ticket where you need to tap again but won't get charged? Or something else?
You could spend two hours in each city trying to decipher the local transport authority's website in each city, and be none the wiser.
Even in my own city (Newcastle) I don't fully know how the buses work! Some have card readers you can tap on and (optionally?) off, but only some operators and some journeys. So I end up buying a ticket from the driver.