The main disadvantage of a digestive system like this is that there is only one opening where the food enters and leaves. Therefore the organism, in our case, the flatworm, must wait for the food to be digested and excreted through the opening in order to be able to eat again.
I think the answer is that they cannot eat continuously. Due to the fact that flat worms have on opening, they will have to excrete using the same hole they eat from. This digestive system is common in simplest invertebrates in a gastrovascular cavity. It is also called an incomplete digestive system.
