This is honestly a myth, all fish are better in planted tanks (and I really mean all, even if the fish have to be physically separated from the plants, and by plants I mean any photosynthetic organisms), the same also applies to a substrate, there are plenty of advantages, and very few disadvantages.
A lot of successful breeders use bare bottom tanks, are OCD about cleanliness, feed with beef-heart and do huge water changes, but there is a reason for this, they have a specific aim (as fast a growth as possible), to stop their fish "growing out of profit". You can think of it as like battery farming chickens.
Once you have a bare tank you are obliged to keep the tank really clean and do lots of water changes, because you don't have any spare filtration capacity or inherent stability in your system, but this doesn't make it the only way to keep them, or the best system.
If you work from the assumption that Discus originate in very clean, nutrient poor clear and black waters, where food is in short supply and they will eat whatever is available, you can then tailor the tank to their requirements, which aren't necessarily the same as those of a commercial breeder. If you keep away from a beef-heart based diet you can feed them a more natural diet, and a lot of problems disappear.
I'd really recommend having a look at Larry Waybright's ("Apistomaster") Discus threads on various forums ("simplydiscus" etc), he is some-one for whom I have the greatest respect.
Here is his Heckel Discus thread at the "cichlid room companion" <
The Cichlid Room Companion • View topic - Heckel Discus "biotope" tank>, and a "Nhamunda Blue" thread: <
The Cichlid Room Companion • View topic - New photos of some of my discus>.
cheers Darrel