Oh c'mon, it's fairly obvious to me that Area training is harder than Aerodrome / Approach training. (I speak having trained as TWR/APR but it looked fairly obvious by watching some of the Area runs of my fellow college mates, and their amount of bookwork, that they had the tougher job).
It's not uncommon for Area students to fail (either at college or real word sectors) and then retrain on Aerodrome / Approach, and succeed. It doesn't quite work like that the other way round !!
Lastly, look at the average validation times of the average Aerodrome sector against the average area sector. There is a remarkable difference. There must be a reason for it !!!
(I emphasise on the word average as there are some very complex TWR and Approach sectors as well !)