The reason is that the plant roots will block the flow of water through the UGF and render it ineffective. Also, the UGF will take nutrients away from the plant roots and starve the plants. Plants and their roots can make cleaning the gravel of a UGF (which you have to do to keep it clear and free-flowing), very difficult.
Other disadvantages to UGFs are that they are difficult to clean even without plants, they need a lot of cleaning because unlike other filters, the dirt is in plain sight, they are quickly rendered ineffective by digging fish or tank ornaments on top of the gravel and they are very poor at removing particulate waste from the water.
However, there are ways around it, particularly if your UGF is just one method of filtration in your tank. You can put plants in pots (very effective and easy) and you can have plants on bogwood (Java fern, Java moss, anubias). For a second filter you can have a cheap and cheerful sponge filter running off an air-pump.
If you got fed up of the UGF but already owned a powerhead, its a simple job to run a sponge or box filter off the powerhead instead or, or as well as the UGF, so nothing is wasted.