Windows Server 2016's VM migration tools broken by a patch

Indeed. Lucky that you have the resources for that. Because, let's not kid anyone - properly testing updates is not cheap.

Because you never know what an update might affect, proper testing needs not just some VMs but full independent, representative hardware. After all, what happens if a software update causes some low-level issue that affects communication to your SAN, but only with specific HBAs. To catch that, you'd need to ensure you're using the same HBAs and same SAN back-end as you use in your production environment.

You also have to make sure that all those components are configured the same and running same firmware and patches and updates and hotfixes as the production kit. And running similar loads because what happens if the bug only occurs when a process or load reaches some saturation point.

And how long do you test for? Because some bugs are only apparent after quite some time. And do you test updates one at a time or in batches?

And just think of all the possible updates - firmware, drives, OS patches, infrastructure updates, application patches - that's a lot, especially if you're diligently testing each one.

And what if you have several, such systems? Each has to be replicated and tested.

Testing is always best practice, but it's also a trade-off and, at some point, testing even potentially critical bugs can be out of reach because those bugs may only occur in very specific situations.