Some of those exit doors weigh in excess of 25Kg and as they have to be pulled inwards first in a twisting movement are really not that easy to open. I kow the first time I did SEP I was surprised however now know what to expect..oh and politely warn the person in the middle seat that I'll probably elbow them in the face quite hard...but don't worry it's nothing personal...and you can hit me back once we're both safe.
But as said above, those persons not fit for these seats should be kept out of them as they will struggle with the hatch and increase the danger for the other passengers. I do know of crew who have been faced with the situation of having obese persons in the overwing seats and having to move them is not easy. That's when you really see how good some of the CC really are when faced with very difficult personal problems, and also to the Capatins in both cases who were both summoned and both fully supported the decision made by the CC in charge.