EU air passenger rights apply:
- If your flight is within the EU and is operated either by an EU or a non-EU airline
- If your flight arrives in the EU from outside the EU and is operated by an EU airline
- If your flight departs from the EU to a non-EU country operated by an EU or a non-EU airline
- If you have not already received benefits (compensation, re-routing, assistance from the airline) for flight related problems for this journey under the relevant law of a non-EU country.
https://europa.eu/youreurope/citizen...r/index_en.htm
If SN gets you to JFK on time then that's it on their part. Any further delay is not in their control. Now, if SN's flight to JFK was delayed overnight (or if your FCO-BRU was delayed and you misconnected to JFK) and you subsequently miss the connection at Newark, then SN would be liable.
https://web.archive.org/web/20190723...for-consumers/ (emphasis mine)
The ruling means passengers of non-EU airlines, who had experienced a delay on the first leg of a flight, which caused them to miss a connecting flight and, as a result, arrived at their final destination at least three hours late can claim compensation of up to €600 euros whether the final destination is within or outside the EU.
I guess in theory, a delay or issue with the UA service could fall under EC261 (since your final destination is on the same ticket ex-EU) but that'd be on UA not SN.