How to get product codes quickly?:

1. Powershell:

Recommended approach.

IdentifyingNumber is the ProductCode (WMI peculiarity).

get-wmiobject Win32_Product | Format-Table IdentifyingNumber, Name, LocalPackage -AutoSize

2. Registry Lookup:

There are always weird exceptions accounted for only by the internals of the API-implementation:

https://stackoverflow.com/questions/29937568/how-can-i-find-the-product-guid-of-an-installed-msi-setup