The ATR72-600 is approved for 120 minute ETOPS (EASA). It is of course a safe plane to operate on one engine, it wouldn't be certified to fly if it wasn't.
I have seen some comments from pilots that it does have excellent single engine climb performance even at max weights, and that it is generally an easy aircraft to fly, but the older models did have some issues in icing conditions but I believe that is ancient history now.
I suspect once the investigation gets under way it will turn out to be something more than a single engine failure.
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