PowerShell v1 through v5.1 have been based on the full .NET framework. PowerShell v6 is based on .NET core which is a cross platform subset of .NET that’s available for Windows, Linux and mac. This has meant that Powershell v6 on Windows is a poor relation of PowerShell v5.1 in terms of the functionality available – for instance the Active Directory module won’t run on PowerShell v6. This makes v6 a backward step for administering Windows. Steps are being taken that will improve Windows compatibility of PowerShell v6.
The .NET core team are releasing a Windows compatibility pack – see https://blogs.msdn.microsoft.com/dotnet/2017/11/16/announcing-the-windows-compatibility-pack-for-net-core/ for what’s likely to be in it.
A number of cmdlets including:
WMI cmdlets
Eventlog cmdlets
PerCounter cmdlets
are ear marked for being ported to PowerShell v6 using the compatibility pack. When this happens and what else is will be included is unsure. You can follow the issue at https://github.com/PowerShell/PowerShell/issues/5603