That's incorrect. There are no roaming fees.

Instead, some telecoms give you a generous number of "credits" for calls, transfer, etc, and sometimes try to limit these to your own country. Then you have to pay overage fees (the same in your home country or elsewhere within EU).

The legality (or ToS language around this) is dubious, but the fact that telecoms try to skirt around the rules doesn't mean they don't work - switch to a better provider.

I just spent a week in another EU country and didn't think for a second about data usage, calls, fees, etc.

Combined with not having to do fx or even carry a passport (national ID is enough), minimal queues at the airport and zero customs annoyance, travelling across continent was as much hassle going to the next town (modulo actual travel time).

(Not to mention the actual benefits of membership, in this case, EU institutions slowly but surely cleaning up house).