The issue i found is that 'OtherIdentifyingInfo' is a collection of objects (i suppose)... so you need to use -ExpandProperty on a select statement, like this:
$helper = $_.hardware.systeminfo | Select -ExpandProperty OtherIdentifyingInfo
$helper variable will get all the objects associated which OtherIdentifyingInfo, so you need a foreach to find the attribute needed. I think you should find what you need on 'IdentifierValue' and 'IdentifierType.Label' Attributes
see if this helps
