Bio balls serve as a form of bacteria housing for beneficial bacteria to live in and colonize. They are fine in its purpose, but they do trap lots of crap and therefore, require frequent maintenance to prevent nitrate buildup. That's the reason why most seasoned reefers would recommend not to use them.
IMO, live rocks serve as the best and the most natural form of bio media. If you must have other forms of bio filtration, biohomme would be a good choice, as suggested by bro bugattilambo.