Encyclopedia of broken UserAgent string detections

In general yes. In some specialty cases user agent detection is fine, e.g. if the application has critical and specific requirements on what software/hardware stack it should be ran on and has been tested for. Just because a feature has been detected to exist in the browser doesn't mean it works correctly, after all. But even in these cases, detecting a non-supported browser should throw a warning, not prevent you from accessing a site.