I had a couple friends go on the Atlantis Project fellowship and they really enjoyed their time there. However, it's mostly just a good opportunity for tourism rather than anything worthwhile on a medical school application. Shadowing doesn't need to be in excess of ~50 hours for a successful application, and should be focused more on American physicians because you are trying to get a sense of what it will be like as an American physician rather than in another country with a vastly different healthcare system. If you're looking to do this for the shadowing component and nothing else, I'd say it might be better to find something else to do. Hundreds of hours of shadowing doesn't help your application, especially when it was done abroad. If you just really have an interest and want to go travel abroad for a little while, then by all means go ahead, but know that it's not going to be looked upon too kindly by admissions committees.