Internal Overflow Sump (IOS) has the benefits of a normal sump, the only difference is that it is located inside the main tank.
One biggest advantage of IOS is that it has the benefits of normal sump, but because there are no external piping, you do not have to worry about your tank water draining out in the case of power/equipment failures or piping connection leakages.
Since the bio media is all put in a straight column, and the IOS compartment is inside the main tank, maintainance is not easy if you need to rinse the bio media. One way to get around it is to keep the bio media in filter bags so that they can be removed easily. This is what most users of IOS systems use, however MOST make the mistake of using constricting bags/mesh bags so that the bio media is not effeciently utilised. Remember that water always follow the path it can easiest flow through. If your media arrangement creates a path where the water can evade the media, it will follow that path! That is also the main reason why some people say that IOS systems are not effective. The reality is, they are not using it correctly!