The downsides of using a wet/dry filter, with bio balls, on my 60 gallon bimac tank are:
1) The wet/dry filter must have a pre-filter (floss, or a sock) to catch particles. I have to replace that filter every few days, to keep the trapped junk from decomposing. The extra maintenance is a pain, plus it means I need to turn off my return pump to feed plankton to my tank. Those who are using live rock with a filter sock have the same problem, so maybe this is a tie.
2) I need to rinse trapped detritus off of my bio balls every six months or so. If I let it build up, it would decompose and be a problem. It's only twice a year, so I put up with it.
3) The wet/dry filter takes up space, which might be a problem for some.
4) Unlike live rock, the wet/dry filter does nothing to reduce nitrate, so I need to have another nitrate export system. I have a 4-5 inch deep sand bed in the display tank, which works well at a medium to low bio load, but not so well when the tank is as full of animals as I like. Also, when the octopus digs, it kills a lot of the anaerobic bacteria, and reduces the effectiveness of the DSB. I think the best solution will be to have a Remote Deep Sand Bed (RDSB), but that also takes up a lot of space.
I put up with these issues because I didn't want to pay for live rock, and because live rock would look wrong in my southern California bimac biotope (local rocks are not porous and provide no filtration)
My nitrates are currently being kept down well by my DSB, and when I install a RDSB, I expect that I'll have no problem at all with nitrate, even if I increase my bio load.
I think that if you have the space under the tank for a wet/dry filter, and a RDSB, and you are willing to use a filter sock (before the wet/dry) and to rinse the bioballs (in tank water) every six months, that you will have no problem with nitrate.