SQL Server 2012 Standard server with 50 instances won't uninstall instances
It will likely be because the setup GUI is trying to enumerate all the instances installed  and failing miserably, running the uninstall from the command line should do it

Running the below command with admin privileges will uninstall the SQL server instance with no setup GUI displayed:

Setup.exe /Action=Uninstall /FEATURES=SQL /INSTANCENAME=MSSQLSERVER /Q


I assume that you only have the SQL Server feature installed, if you have others you want to remove, you can just add them in a comma separated list of values, such as /FEATURES=SQL,RS,Tools

For different instances, just replace MSSQLSERVER with the name of the instance you want to remove.

Command line reference for setup.exe is available here:

Install SQL Server 2012 from the Command Prompt