Dangerous enough but like anything a managed risk. A lot of light aircraft are ferried over to Europe all year round. They often pass through Ireland. I can only remember one accident a few years ago when a light single went down in the Atlantic just short of Shannon airport.
Aircraft engines are really reliable, and it is often said that the engine doesn't know it's over the sea. The variable weather is a big issue though.
It is something I have often thought I'd like to try, maybe not so much since I acquired two kids.
I have flown a light single over the sea. The Irish sea and the English channel. When you are out of gliding distance of land and you see the rough grey ocean below you. Your mind does begin to hear things with the engine. The 'auto-rough' syndrome. Try imagining that for hour after hour over the ocean. I flew with one pilot in a Cessna 172 back across the sea on a gloomy winter day, quite low because of the cloud. I quickly noticed his knee was shaking as he checked and re-checked the temperature and pressures and the mixture and the RPM gauge.
I remember once been woken in my room in the Loftleidir hotel in Reykjavik, Iceland by the sound of a light single being run up on the adjacent airport. It was a Cirrus, obviously on a ferry flight. I must say I didn't envy that pilot. It was still dark, about minus 5 celsius with snow on the ground. When I thought about his onward flight from Iceland, probably through Scotland and onward. I just shivered at the idea and went back to bed. He had already crossed part of the Atlantic but there was more to come. I much preferred the cosy 757 that brought me there.
I do think that the biggest risk is the weather, the engines are reliable, the navigation systems are almost faultless, but you can't control the weather. The risk of flying into something nasty is always there.
I think a lot of Cirrus's bound for Europe fly the Atlantic. I haven't heard of one go down yet.
