i think filterless tanks if setup right can actually be far safer and more stable for your fish. if 70% of your beneficial bacteria are housed in a filter media you can devastate a tank if something should happen to it. where as with a "filterless" the bacteria happily colonize the gravel, and your chances of crashing a tank are far less.
you can do flow or no flow depending on your size and bioload. more heavenly stocked tanks with larger volumes of water would probably need some help from a pump. in my setups usually 10s and 20s i never use any flow. it seems to work fine for my 50 too. but that will also depend on the fish. if i where setting up a hillstream loach tank it would look like the inside of a washing machine.
one thing also to think about is a deep sand bed. it's easy to provide an area for aerobic bacteria, the same that any standard filter on the market utilizes to eliminate ammonia/nitrite, but something most don't realize is that you can also have a population of beneficial anaerobic bacteria a few inches under your gravel that will work opposite to the other bacteria and eliminate nitrates, reducing the need for water changes.
Doesn't a deep sand bed go bad over time with no stirring of the substrate.