The first disadvantage of a metal detector is that it may give a false alarm. This may happen if someone is carrying any sort of metal object on them (watch, phone, toy, jewelry). This may cause panic for no reason and cause the person to be embarrassed. This is seen as a major inconvenience for both the person and staff.
Another disadvantage is that the metal detector isn't really effective with people who have had to get a pacemaker or other types of replacements such as a knee replacement. It may also be unsafe for these people. In most cases, these people avoid going through metal detectors.
The final disadvantage of a metal detector is that it may interfere with certain personal devices. For example, it may interfere with a magnetic recording device. An example of this is a cassette or CD. It also interferes with other things such as a cell phone, forcing people to turn it off before walking through.