1. There's always a friend's computer. 2. They're a form of censorship. 3. They give you a false sense of security. 4. Kids resent them -- and you. 5. Kids can defeat them. 6. They catch too much. 7. They don't catch everything.
Most parental control programs use a combination of filtering techniques to block access to unwanted sites. But each method is vulnerable, and none promises 100% accuracy. Text-based filters can't really determine the context of words or phrases, so they can block access to perfectly acceptable sites. Words like "sucking," for example, might get caught in the filter and prevent your kid from researching, say, mosquitoes.
(Source: Common Sense)