Powershell and SCCM - Unable to make a INNER JOIN query with CIM (__GENERIC CLASS error)

Question

I am currently working on a Powershell module to help my colleage and I to works more "seemlessly" with SCCM and script (since the SCCM Cmdlets are way too slow, and therefore unusable).

In this module, I have created a function called Get-CMCollectionsAndFolder which retrieves all the collection from SCCM (using the class SMS_Collection). I also tried to add to that list the FolderName and ContainerNodeID of those collections, as indicated in the SMS_ObjectContainerItem class.

All of my module requests are done using CIM, but for some weird reason, I can't make the JOIN query written below working with CIM, only with WMI.

So, if I run this :

Get-WmiObject -ComputerName $computername -Namespace root\sms\site_$sitecode -Query "SELECT SMS_Collection.*,SMS_ObjectContainerNode.ContainerNodeId,SMS_ObjectContainerNode.Name FROM SMS_ObjectContainerItem  INNER JOIN SMS_Collection ON SMS_Collection.SiteID = SMS_ObjectContainerItem.InstanceKey INNER JOIN SMS_ObjectContainerNode ON SMS_ObjectContainerNode.ContainerNodeID = SMS_ObjectContainerItem.ContainerNodeID"

I'm getting an array of object, containing thoses properties :

TypeName : System.Management.ManagementObject#root\sms\site_IP3\__GENERIC

Name                    MemberType    Definition                                                                                   
----                    ----------    ----------                                                                                   
PSComputerName          AliasProperty PSComputerName = __SERVER                                                                    
SMS_Collection          Property      System.Management.ManagementObject#SMS_Collection SMS_Collection {get;set;}                  
SMS_ObjectContainerNode Property      System.Management.ManagementObject#SMS_ObjectContainerNode SMS_ObjectContainerNode {get;set;}
__CLASS                 Property      string __CLASS {get;set;}                                                                    
__DERIVATION            Property      string[] __DERIVATION {get;set;}                                                             
__DYNASTY               Property      string __DYNASTY {get;set;}                                                                  
__GENUS                 Property      int __GENUS {get;set;}                                                                       
__NAMESPACE             Property      string __NAMESPACE {get;set;}                                                                
__PATH                  Property      string __PATH {get;set;}                                                                     
__PROPERTY_COUNT        Property      int __PROPERTY_COUNT {get;set;}                                                              
__RELPATH               Property      string __RELPATH {get;set;}                                                                  
__SERVER                Property      string __SERVER {get;set;}                                                                   
__SUPERCLASS            Property      string __SUPERCLASS {get;set;}                                                               
ConvertFromDateTime     ScriptMethod  System.Object ConvertFromDateTime();                                                         
ConvertToDateTime       ScriptMethod  System.Object ConvertToDateTime();

I then only have to "expand" the SMS_Collection and SMS_ObjectContainerNode property to get my info, which isn't that bad.

But when I try the same query and replace Get-WMIObject with Get-CIMInstance, I obtain that error :

Get-CIMInstance : The WS-Management service cannot process the request. The class __GENERIC does not exist in the
root/sms/site_SMS namespace.
At line:1 char:1
+ Get-CIMInstance -ComputerName $computername -Namespace root\sms\site_ ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (:) [Get-CimInstance], CimException
    + FullyQualifiedErrorId : HRESULT 0x80338000,Microsoft.Management.Infrastructure.CimCmdlets.GetCimInstanceCommand

I suppose there something obvious that I'm missing, but I clearly don't see. Can someone give me a solution and/or the reason of this behavior? I'm a noob in WMI and CIM, only working with those in the last 3 weeks so I still got a lot to learn.

Thanks!