To clarify, in most cases this means "pax remain on the aircraft for an extended duration until the flight is able to continue". An extreme case, but for example this flight spent over 12 hours on the tarmac overnight in the winter (with pax aboard) after diverting to YYR.

If the flight can safely make it to the US, especially a major US station, it will be much better for the pax. Can't say why BOS versus EWR - I am sure the crew made an informed decision.