I agree that a UGF
can be a good option as long as 1) it's used with a powerhead or two, 2) The tank is heavily planted to use up the nitrates they put off, and 3) the fish (over-stocked or not) aren't over-fed.
In short, I don't think they're a good idea for beginning aquarists. There's too many ways for them to go bad. It's my opinion that they're more trouble than they're worth. The best filtration IMO for a small tank is a good power-filter (HOB, backpack, whatever) and/or a canister. I use both.
If you've had a UGF running for 10 years I suggest if you were to look I'd almost guarantee there's a huge build-up of mulm (sludge) down there. The fact that your tanks are heavily planted is most likely what's keeping the system in good balance. It would be interesting to know your nitrate readings. I don't have any experience with planted tanks and it would be neat to see how much of the nitrates they utilize.
My most recently put up tank (125 gallon) has a under-gravel jet system and an FX-5. The UGJs blow waste off the sand and the FX-5 sucks it up. The sand keeps waste on top of it rather than letting it go down under and rot. It's the nearest to a perfect system I've ever had filter-wise.
Really, as long as the tanks are filtered properly one way or another it's all good. I just think that my way is better for a beginner.