It does provide some great ways to get rid of ammonia and nitrite. Also, the upwards flow of water through the gravel pushes solid waste into the water column so it's easier for the canister filter to remove the solid waste. You shouldn't get cloudy water or grubby gravel, assuming of course you clean the mechanical filter media. You can clean the canister filter as often as you want without worrying about killing bacteria, because there will always be lots of good bacteria in the gravel. Some Con's to reverse underwater filtering. Two of note. Firstly, they're essentially incompatible with plants, or at least plants with roots (they're fine with floating plants and epiphytes such as Java fern and Anubias).
Secondly, they only work properly if the gravel bed is more or less flat and open. You can't cover much more than, say, 15% of the substrate with rocks, and if the gravel bed is particularly thin anywhere, the water will mostly go through that area of least resistance, by-passing most of the filter bed.
all of the are from: http://www.wetwebmedia.com/fwsubwebindex/fwugfiltfaqs.htm
It is possible to avoid the biggest problem of UGs, their excellent mechanical filtration potential, by reversing the flow. That is, instead of pulling the water down through the gravel, you push it up through the gravel. You also use a sponge pre-filter on the powerhead, so it is pushing pre-filtered water down the riser tube, out under the plate, and up through the gravel. This is reverse-flow undergravel (RFUG) filtration. Several brands of powerheads can be adapted to this, but I only have personal experience with one, Penguin, which offers an accessory kit to convert their conventional pump to reverse flow. The kit has an elbow attachment to mate the pump output tube to the riser tube and a sponge assembly for the intake side of the pump. This is not expensive, and being a lazy man, I haven’t tested any other technique since these came on the market. The sponges are easily removed for rinsing under the tap. There is no concern with preserving any nitrifying bacteria in residence in the sponge, as the gravel bed is to perform that service. The sponge can function purely as a mechanical filter, and be rinsed weekly, biweekly, or whatever your water change interval may be. I like biweekly at least, weekly is better, to get waste out of the tank before it is completely digested in the system. The gravel is still hydrovacuumed, but you will be astonished at how little material comes out of the gravel compared to conventional flow UG, or to conventional substrates.