In my years of gathering info from forums, websites, other aquarists' experiences etc, I have come to the conclusion that this whole business about UGFs being bad for planted tanks is pure crap.*
I put an asterisk here because I am not stating this from personal experience or scientific experiments. But I know of many situations where aquarists have maintained wonderfully planted aquariums for many years with UGFs, and yes, they were done so with several heavy root plants.
Some people have argued that the UGF actually helps these plants because it brings nutrients and circulation of fresh water to the roots.
That out of the way my opinion of UGFs are as follows:
Pros - massive place for bacteria colonization, absolutely no cost for media replacements, can be completely silent, safe for fry tanks and slow swimmers like bettas.
Cons - absolutely disgustingly dirty. keeps all of the dirt, mulm and filth in the tank and thus must be constantly maintained by turning the gravel bed and heavy vacuuming; which in the end still doesn't result in the removal of all the accumulated waste if the tank has a large number of, or very messy inhabitants. Doesn't allow for any sort of customization (can't add any specific chemical filtration like peat, carbon or phos-zorb) and just generally not worth it in the long run.
Verdict - if you want to save money on filter media, it's your best choice. But other than that, completely unnecessary. There are other and better choices for almost every purpose imaginable.