If you are talking about CL Free (aka chlorinefreeusa.com), this appears to be a combination of copper ionization for the bulk pool water along with ozone (or active oxygen) for water that gets through the filtration system. The copper prevents algae growth, but at the risk of metal staining which is why the pH needs to be kept low and metal ion concentrations monitored carefully. The ozone/oxygen helps oxidize bather waste.

The problem is that there is no fast-acting sanitizer in the bulk pool water. Copper simply does not kill pathogens very quickly. Chlorine (at typical pool concentrations we recommend) kills most bacteria in around 1 minute while silver ions take around 10-20 minutes and copper ions take around 40 minutes. It's just enough to prevent uncontrolled bacterial growth, but not to kill quickly. Also, metal ions are not effective (at pool concentrations) at inactivating viruses. For example, this paper shows that copper ions do a 90% inactivation of Herpes Simplex Virus in 30 minutes at 100-200 ppm, but that is far, far higher in concentration than found in pools (copper is usually < 0.3 ppm in pools). This paper shows that silver ions have virtually no effect on vacciniavirus, adenovirus, VSV, poliovirus, HVJ, but that with herpes simplex virus there is a 5-log kill in 60 minutes (roughly a 90% kill in about 5 minutes), but at over 3200 ppb compared to the usual limit of 20 ppb to prevent silver staining.

As Carl says, it's a trade-off, or as I like to put it, it's a spectrum of risk. In a residential pool, the risk is lower since you generally aren't having lots of people over with diarrhea or other illnesses swimming in your pool, but this sort of system is not allowed in a commercial/public pool because it would not prevent transmission of disease from person-to-person.

Richard