These security issues are in no way a disadvantage of the language itself.
Cross-site scripting (XSS)
Cross-site request forgery
Browser and plugin coding errors
Sandbox implementation errors
Malicious/Vulnerable libraries and frameworks
Again these are in no way a disadvantage of the language itself.
Potential loss of web page functionality due to user disabling JS script execution. This cannot be validated by the server because JS is client-side only.