Under gravel filters work very well but need to sit flat on the tank base with the substrate on top. You can't have fine substrate, like sand, as it gets sucked through.
You also need at least 5cm of substrate on top of the filter plate if you wish to successfully grow plants.
I would use UGFs in a small tank, up to around 60cm wide but not on larger tanks unless I also had supplementary filtering, or several outlets along the plates.