Prior to the popularity of MDN and campaigns like W3Fools (q.v.), the articles on W3Schools were very bad from a web standards perspective (<marquee> and VBScript levels of bad); from a programming perspective, their PHP articles (which largely date from that time) are still incredibly shoddy. Their unwillingness to relinquish the unwarranted authoritative tone or at least visibly disavow any connection to the W3C (who asked repeatedly), coupled with their domination of search results, at a time of a standards push (remember validator buttons?) and multiple high-quality documentation efforts (later folded into MDN; Opera in particular had a good course) just plain got on people’s nerves.

They’re a valid reference, just a low-grade one, and using a low-grade reference is rarely a good tradeoff. They also used to be worse and never quite shook of their reputation.