I would avoid a proving flight on any aircraft, but once it is in service it is as safe as the older ones.
The twin engine thing is slightly different though. Engines do shutdown in flight occasionally, and while aircraft are perfectly capable to fly with one (or two for the 4 engined aircraft) engines, being in the middle over an ocean does limit your options somewhat if another engine goes down. Hence the ETOPS regime - the more reliable the engines are proven to be, to further away you are allowed to fly from diversion airports.