Metal detectors use different technologies with different options and performance settings, depending on the targets created. In other words, there is no such detector that suits all the targets possible. There are only five kinds of such devices.
First of all, you have the metal detectors which are used for detecting coins, relics and treasures, this being a very general spectrum. Then, you have the devices which are used for detecting gold and the ones which are used for finding things which are underwater or on beaches. There are also detectors which work at very low depths and, last but not least, there are industrial detectors.
Most of these devices are conceived so that they offer the best results for a certain type of search, but they can also be used for other kinds of targets without offering the best results such as in the case of the targets they were created for. For instance, the best detector used for detecting nuggets is very sensitive to very small pieces of gold, being highly performing in this field, but it not as efficient when it comes to detecting coins because they detect every small piece of old iron. The devices which are created for detecting coins are not designed in such a way that they can detect small pieces which only have the size of a rice grain and they deal with iron better than the other kind of device.