On CentOS and RedHat the tests in test/powershell/Modules/Microsoft.PowerShell.Utility/WebCmdlets.Tests.ps1

Invoke-WebRequest tests
  HTTPS Tests
    Verifies Invoke-WebRequest Certificate Authentication Successful with -Certificate
Invoke-RestMethod tests
  HTTPS Tests
    Verifies Invoke-RestMethod Certificate Authentication Successful with -Certificate

Fail with the message The handler does not support client authentication certificates with this combination of libcurl (7.29.0) and its SSL backend ("NSS/3.16.2.3 Basic ECC").

This is referenced by this corefx issue: https://github.com/dotnet/corefx/issues/10146 which is still open (as of Nov 17, 2017). We need to document when we release.