Can't uninstall Microsoft SQL Server 2016 RC3
Thanks to Aaron Bertrand, I was able to resolve the issue following the manual steps outlined in his blog:


Fun with software : uninstalling SQL Server 2008 R2 Evaluation Edition


...in spite of the title, it's not just for 2008 R2 or Evaluation Edition.

In case the link ever stops working, the essential steps are:


Locate SQL Server related GUIDs under:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\

Also look for GUIDs in folders under Uninstall called e.g. "Microsoft SQL Server 2008 R2"
Run for each item:

msiexec /x "{guid}"