Ianice, welcome to the Planted Tank!
It's an interesting theory which hasn't proven to be long-term stable. Several issues:
The slow flow through the substrate is still too fast, and the oxygen-rich water makes trace elements unavailable to plant roots. Not much of a natural fertilization. Sure, you could reduce the flow to almost nil, but the many plant roots will further block the little flow and finally there won't be any, so why go through all the troubles?
Roots find their way down into the UGF cavity, clogging it up over time. Again, it works for the unplanted area, but what's the point? Fishies and plants don't mind a little turbulence (water movement), and in a planted tank, biofiltration isn't that much of an issue anyway.
For root feeders, adding some kind of fertilizer tabs to their proximity is almost necessary when using gravel. The reverse UGF will slowly move that into the water column. There isn't much control in terms of where the actual flow occurs, but for sure it ends up in the water instead of feeding the roots.
The normal tendency of mulm/fish poop to disappear into the gravel will be reversed by the reverse UGF, instead of disappearing, it will collect on the surface (I think). A big sponge in a canister filter has a better holding capacity and is easier to clean than an area of gravel in your tank.
You might end up having radiantly healthy plants either way, but with the UGF, you actually hear a little time bomb ticking... a place that slowly fills up with yucky muck, although... maybe you could reverse the flow and suck out the muck once in a while... Maybe you should just try it!