In truth, bioballs properly maintained and serviced are just as effective, if not more so than live rock at the process of converting ammonia into nitrite and nitrite into nitrate. Unlike live rock, they will not convert nitrate into nitrogen gas, as they lack the anaerobic area of LR.
Proper maintanence and service requires that a portion (generally 1/2) of the bioballs be removed monthly and rinsed in freshly mixed saltwater and then returned to the sump. Many fail to keep up with the required maintance, and because of trapped organic debris, their systems nitrite or nitrate level rises.
Many aquarists remove the bioballs, and replace them with live rock rubble, thinking that they are circumventing the maintance issues - and for a time period they do. In reality however, the pores of LR can become filled with the same debris that caused problems with the bioballs, and the same issues can occur with LR rubble. Generally, if you replace the bioballs with LR rubble, it is still recommended that you occassionally remove and rinse the rubble to prevent clogging.