Many are now thought to be Carcinogenic. Copper and zinc are both safe for drinking water as well as spa water in quantities much greater than necessary for purification. The need for high levels of chemicals is eliminated. Also eliminated are the strong chemical odors, damaging effects on hair, skin, fabrics, liners and pumping equipment. Furthermore, you eliminate the need for frequent trips to the store to purchase heavy containers of chemicals.
Chemical storage is no longer a problem.
In most cases, the electrodes will last five to ten years and are relatively inexpensive to replace. The only other cost is the electrical consumption which is only a few dollars a year. You will need to test the ion level every few weeks and maintain proper water balance. Bromine is more expensive than chlorine and still burns the eyes and skin. An ionizer costs less up front, but the electrodes do wear out in 5 to 10 years and will eventually need replaced.