in order to do a lot of things, an antimalware solutions needs to have relevant modules (antivirus, antispyware, antiadware, desktop firewall, desktop intrusion prevention, device controller, application controller, tamper proofing, etc.). a product doing things in half-measure may come across as light.
also, the USB autorun issue depends on the way the solution is configured. in order to reduce resource utilisation, configuration is set to check only when accessed, which could lead to the reported situation.
having said the a/m, any antimalware solution is as good as its last update, the various proactive, preventive & heuristic mechanisms, etc., for detecting new and/or unknown malware being in place notwithstanding.