Long Term WPMU Management?

For those of you running a WPMU Network installation with many users, how do you manage your themes, plugins and WordPress updates? I know the team here at WPMUDev does a good job at updating their plugins and themes to work with every version of WordPress, but what about outside themes and plugins?

One of my concerns in setting up a network that other people depend on or even pay me to provide, is the methods and resources needed to maintain compatibility between the various themes, plugins and WordPress itself. How to update to the next WordPress version while ensuring that the Premium WooTheme one of my users has selected doesn’t break?

Should there be a test environment and testing happening before every update? That means testing every plugin and theme not provided by the WPMUDev crew?

Thanks in advance for any insights you can provide.