I suspect this is an issue because we hit the 50 instance max limit of SQL Server. At the limit it won't install more instances, naturally. However, it seems it won't UNinstall them either. That's worse.
It hangs at the "Select Features" dialogue when you hit next after selecting the SQL Engine for the instance.
I didn't find a lot of info about this. The most interesting hit was this: http://social.msdn.microsoft.com/Forums/sqlserver/en-US/24a1e5f3-25f3-48c8-973a-4c6a18578e42/trying-to-uninstall-1-of-50-sql-server-2008-r2-express-hangs-on-please-wait
It also suggest using
setup.exe with some parameters to uninstall the instance, but I can't get it to do so without starting the GUI and into the normal uninstall which fails.
Elevation does nothing. Reboot doesn't help. Logs only say "canceled by user", because I have to kill it in the end. Nothing obvious prior to that point. Validations are all green.
The setup says it is SQL Server 2012 SP1 - all instances are at 11.0.2100.
I' choosing to to answer/close my own question since Microsoft has closed the ticket with the status of "Won't fix."
Hopefully they will have fixed this for SQL Server 2014, but I have not and do not plan to test that myself, nor do they provide any additional comment on closing the ticket.