I've had two FOWLR
tanks with bio-balls in the built-in filtration in the back (not a sump underneath type). Both tanks had ZERO problems with nitrates...ever To me, I do not understand how live rock/rubble would catch less than the balls. I'd think lots of small rock would catch all sorts of detritus.
I have mine set up slightly different than factory now, versus my completely stock setup many years ago. Now, the water comes from my tank into the first of three chambers. Nothing exciting in there. The water goes from that chamber into the second chamber from the top. I believe that normally has a plastic trickle plate where the water drips down into the bio balls. I replaced that plate with a big sponge. That catches all the waste and I clean that out every week or two. Maybe that "catching the detritus" saves the bioballs from becoming nitrate factories? That water goes into the third chamber from the bottom and is pumped back out into the tank. My Trates are always at close to zero.
I've considered adding something like cheato into the third chamber, but not sure if it would get enough light and whether I should mess that is working for me now.