Going from memory, ClO2 byproducts include chlorides, chlorates and trihalomethanes, all of which in large and chronic doses are harmful. Simple chlorination (using sodium hyplochlorite, i.e., bleach) has all sorts of byproducts too. But of course ClO2 use on the occasional camping trip really don't present any sort of realistic risk, especially compared to the viral, bacterial and parasite infections avoided by treatment.
Generally speaking, such disinfection byproducts result from oxidation of organics, so the cleaner the source water the fewer the byproducts, resulting in an even further diminshed risk.
Anybody planning an extended trip and chronically using poor quality source water should perhaps look at two-step treatment such as Cl02, then carbon adsorbtion. But weekenders and week-long hikers probably needent worry at all.
p.s. The comment on the rarity of municipal ClO2 use in the States is correct. Treatment here is mostly limited to chlorine and chloramine.