Actually the truth is. Trashy sites multi freq can have disadvantage over single freq I.e. higher conductors mask by lower conductors. Machine though using single frequency needs to possess lower single few ops like 5 kHz approx.
Multi frequency biggest advantage is more accurate ID of target at deeper depths. Plus they do better calling nonferrous targets nonferrous targets at deeper depths (likely mid depth) in higher mineralized soil- even though the MF unit could be in error ID wise. Single freq unit here could call the same target ferrous.
One is getting part of the best worlds really using multi freq. sweeping over higher and lower conductive targets.
Keep in mind most MF detectors now sold have multiple modes. The freq used per mode can change or the frequency weighting can change.
So one cannot use a currently sold MF unit in one MF mode and necessarily be all they can be at detecting various sites, various targets of interest or various goals.
In sites with lot of iron. Say relic hunting. Single freq can have advantage and MF can have advantage per coil sweep direction over detecting scenario . Lot of detectors having MF ops do possess single freq ops sold today.