[Delphinus]

The biggest fallout is the cost of salt.

It's actually not that new of an idea. I'll have to check back some old threads but there are several articles are written about the idea of water changes and how basically there is a point of diminishing returns with them, and basically, not unlike feeding, "smaller more often" is better than "larger less often". Some systems in fact incorporate constant water changes - ie, there is always a steady stream or drip coming in and it just overflows into the drain. Some captive breeding systems are setup this way. You would have to measure your salinity though regularly and manually compensate for the drift.

To be honest though I'm not real sure you'd get a "true" ULNS out of increased water changes. Your rate of export has to match or exceed your rate of production and since one is steady state and one is stepped, I don't think you will truly get there. I think the tank will certainly benefit, but it's not an as aggressive method of nutrient removal as the bacterial based ULNS's.

I think it really comes down to time, energy and cost. Otherwise it's pretty much just win.