Medicine Degree not compatible in other countries?

I want to study medicine at university and potentially specialise in either psychiatry or go on to become a neurologist/ surgeon. I was wondering whether or not I will be able to travel and work in different countries with an American degree as this would be important to me. I have heard that you have to take loads of exams to go form one country to another. Is this true and how hard are these exams to do well in? Are they very time consuming and a hassle to prepare for? (so time consuming/ hard that I would consider not taking the exam) Thank you!