This fails (and wont let me add /L for logging) and displays the help popup:
Start-Process -FilePath msiexec -ArgumentList /i, "C:\app\install\mariadb-10.0.14-winx64.msi", INSTALLDIR="C:\ipw\bin\MariaDB 10.0\", PASSWORD=sekrit1234,STDCONFIG=0, SERVICENAME=MySQL, /qn -Wait
i.e. im trying to install it as a service with the specified rootpassword, and have the containing script wait until finished.
FYI, this command works, it runs just before in the same powershell script:
Start-Process -FilePath msiexec -ArgumentList /i, "C:\app\install\PHPManagerForIIS-1.2.0-x64.msi", /qn -Wait
i.e. msiexec works to some extent.