Here's a very good list of pros and cons to using UG filters. https://tropical-fish-keeping.com/tr...l-filters.html
From a personal point of view, having used them for my marine tanks before the wet/dry filtering system came to hobby, they served a purpose but were a pain to maintain since the "stuff" you want out of your tank is under the filter plates so not easily accessible. As for my freshwater tanks, I stopped using them after I did a service call on a Discus tank with one and found, literally, a pint of tubifex worms growing under the plate. No amount of gravel cleaning alone would have kept that tank clean. After that, I switched back to outside filters and either sponge filters or box filters.
Hope this helps.