Undergravel (UG) filters did revolutionize in-home aquaria. Before their common use, one had to have a heavily planted tank and stock extremely lightly by today's standard. Effectively, the filtration was performed by the plants. UG filtration allows the entire gravel bed to become the home to all the beneficial bacteria. Lots of surface area, a significant amount of flow, and some mechanical filtration is perfomred as the larger particles get sucked down into the gravel and kept there.
But they can probably be considered obsolete too. You had to gravel vac part of the tank consistently. If too much gunk built up in one point, the beneficial bacteria are blocked from the flow. Bacteria blocked from the flow are in a low oxygen zone, meaning they become anaerobic, and may excrete harmful chemicals. On the other hand, the gravel bed could 'rathole' in that a channel could form where in the water could bypass the gravel bed -- and the ammonia would not be reduced to less dangerous wastes. But, you could not over clean the gravel, either, as you might disturb and/or vacuum up too much beneficial bacteria causing a mini-cycle. So, you have to clean the gravel some, but not too much.
There is nothing inherently wrong with UG filters, I just think that there are better alternatives out there, such as the revolutionary idea of the filters with a bio-wheel. The great thing about the bio-wheel, you know exactly where the beneficial bacteria, so you know where to keep safe from dechlorinated water and medications. So long as you keep your bio-wheel safe, no mini cycle.
