Data roaming advantages: You get data when you roam off-network (i.e., not on your own carrier's network).
Disadvantages: Each carrier has their own limit on how much data they allow you to use when roaming. Roaming means they have to pay the other carrier for the data you use and that comes at expensive rates. If you roam excessively, your carrier can terminate your service entirely.
Smaller carriers may still charge this, but generally it's part of your service with your carrier. This is why the option to prevent Data Roaming on the phone still exists. If you turn it off, the phone will not use data when roaming - and you don't get charged.Thanks.
Do you pay extra for data roaming ?
No, it's part of your service with your carrier. But they limit you.
I'm with T-Mobile and the current plans limit data roaming to 200mb. I'm on an older plan so my data roaming limit is probably less. But after 200mb on the current plans T-Mobile will stop you from using data when roaming on another network.
This is per month.
Some carriers used to charge extra, some still might. This is why the option to prevent Data Roaming on the phone still exists. If you turn it off, the phone will not use data when roaming - and you don't get charged.