Lots of hot air here as usual from the misinformed. There is far more possibility of an incident attempting to land in an unsuitable area than a winch rescue by a well-trained crew who are aware of the consequences of an engine failure .
The OEI (and I have only heard of one engine failure in a 139) capability of the 139 is good provided that the pilot has some idea of what will happen OEI. The result will be either no effect and the winch can be completed OEI, a flyaway that will involve some height loss or a committed landing to whatever lies beneath the helicopter. These can be calculated fairly accurately using environmental considerations, the PI figure, the Perf Data drop down height loss and pilot experience.
Crew judgement on the options of landing vs winching, a good pre-winch briefing of crew actions during an engine failure and good situational awareness makes for as safe a winch as possible.
Of course there is no substitute for experience and constant training. Our operation completes 2 hours training per crew every day and this usually involves winching day and night to decks, cliffs, wets or drums. OEI hover can be replicated well in the 139 simulator and we make good use of it as part of 6 monthly OPC training.