The NOS is only one facet. You can simplify your system by going with a single vendor or you can be prepared to hew a trail into middleware. Now examine messaging, the No. 2 network application. The majority of today's network messaging systems could hardly be considered enterprise. Hewlett-Packard's OpenView is robust, but mainly lives in the Unix space.
Applications have a long way to go to reach the peaks of reliability.
Reliability is also a function of system complexity. A system with few moving parts is less likely to break down than a system with many moving parts. That's common sense. A system with fewer dissimilar components is also less likely to break down. Networks are terrifically complex simply because they are made of hundreds of different parts from hundreds of different companies.