In most cases you won’t be able to use the “UninstallString” value within the registry to uninstall the software. In most cases the software vendor puts something in there that will show a UI and will not automatically uninstall the software in a way that can be done any way but by hand.
You can generate uninstallers for all MSI applications rather easily. I have about 6000 of them here: https://github.com/jgstew/bigfix-content/blob/master/fixlet/Tasks_UninstallApplicationsMSI_Windows.bes
These were automatically generated using some REST API automation. If your need is just to be able to uninstall MSI applications, then the automation I have will build the BigFix tasks to uninstall them quite easily.
Generally to uninstall MSI application, you want to use the GUID of the uninstall key directly and build the MsiExec command yourself with that GUID so that you can reasonably ensure that you are executing an uninstall command that can be automated without launching a GUI or rebooting the computer silently. EVEN SO, you still won’t be able to be certain that this is the case because some applications with require extra command line parameters to uninstall silently or to prevent reboots or to prevent launching of Internet Explorer at the end of the process. The only way to know is to test each one very carefully.