I've been using the PowerShell workflow activity in SP2010 for a while, and was in for a surprise when I can install it on SP15, but both Designer 2010 and Designer 2013 won't serialize the @#^@ thang so I can edit/view the powershell. I get a "the
list of workflow actions on the server references an assembly that doesn't exist"
Likewise, the upgraded workflow doesn't run :( (Doh!)
I'd rather not fire up a Azure cloud to run some new-fangled workflow just yet (I'm sure we all will be doing so soon!)