Firstly UGF is no good with sand the gravel should be around 3-4mm..
You can keep more than bettas with them, before I went marine (around 7 years ago) I had no trouble with UGF filtration and raised a couple of healthy discus with an UG filtered tank, without issue... The tank was running for over 2 years before I went marine, and when I broke down the tank as I used it for marine and sold the discus, the UGF was not clogged up at all and my plants (anubas nana.) root systems were under there, but the plants were growing well...
I don't understand the anti-UGF sentiment here, it seems very strong and the problems reported mainly sound like they had UGF, but never cleaned the gravel, hence the ineffective filter? If I never changed the cartridge in a hang on filter it would clog up eventually too.... UGF are very effective if maintained, like many other filtration systems..