I've seen Cory from Aquarium Co Op talk about it before, he tested it out to see how effective it really is, how/why it might have fallen out of fashion.Ye Gods and Little Fishes!
It's been yonks since I last saw someone promoting undergravel filtration!
I think it was in one of his streams though, and I might be misremembering what he said so don't quote me on it - but I think he said that it worked really well when he tested it, talked about why that might be, and suggested that filter companies might have something to do with it falling out of fashion... the marketing to push people to buy other filters rather than stick with their decade old undergravel filter that was still working perfectly, you know...
Again, don't quote me on it, just what I think I remember him saying and suggesting. I'd give one a whirl if I had one and the right tank/stocking for it. I know that the tank I inherited from my dad is very old school, with a huge thick hood on it, and it had in-hood filtration boxes which worked brilliantly for years. Not a pretty hood/tank combo - I like that we can get things like rimless tanks nowadays. But those large filter media boxes held so much media that you could really overfilter that tank if it had a powerful enough pump. But can't get the right sized pump and fittings to repair them.