E.g. A depends on B. In a large update, B updates, something unrelated fails to install and the update fails, and now A doesn't run because it wasn't upgraded and the still installed old version of A depends on the no longer available previous version of B.
In NixOS, you either enter the new environment with all packages updated, or you don't. In the situation where you discover run-time - not install-time - errors, you can simply roll back to the previous set of packages. If the new packages cause a system crash, you can enter previous states from the bootloader.
Now none of this magically solves the software defects themselves. But it gives you a consistent system state that can be transactionally upgraded, or reentered at any time. And then you can raise the appropriate bug tickets.
Your system configuration is in a single file and the need to reconfigure different services in a particular order is gone.
Also your dependencies are all nicely pinned with Flakes. This makes importing arbitrary dependencies - open-source or proprietary - a breeze. This has removed any previous appetite we had for a monorepo, which was primarily to ease access to our other libraries.
It's all very nice, once you are over the not insubstantial learning curve that is.