Explaining the concretes would be a lot of unpleasant detail. The why is far more interesting.
I am very good at "tunnel vision". Taking a thread and following it in depth. Completely learning a system in depth so that anything that comes up, I know exactly where to go and what to do.
I am weak on "peripheral vision". Keeping track of 15 balls in the air, and operating with limited information about each. Being effective with limited context about each individual system because there isn't time to learn any of them in depth.
Tunnel vision is a virtue in a software developer. Peripheral vision is essential for a sysadmin.
Now imagine a person with poor peripheral vision trying to learn a system as big and complex as Google's. And being responsible to support 15 different pieces of software written by 15 different teams so that when anything went belly up with any of them, you can trouble-shoot and get it running again.
Nothing particularly bad happened, but I also wasn't accomplishing the job to the standards that Google wanted in that role. And I was not the only person on my team failing in that way.
Ideally it would have been my manager's job to say, "I recognize that this person isn't working out here, is there a better role for them?" That didn't happen. Several months later my manager got fired. I was privately told that my situation was a trigger, but I don't know details.
The fact that my situation is fairly common strongly suggests that the ultimate failure was organizational and systemic. I don't fault Google for hiring developers into their hybrid developer/sysadmin role. I do fault them for not having an explicit onramp/reconsideration process to mitigate the risk that they create by doing so.
Well that's just the thing - an organization that has a "system" or "process" for handling employees and their work is by design less than flexible. In a small to medium sized business, you might have 3 completely separate roles, and others might pitch in as needed. But at a large corporation, it's simpler and more efficient for them to have one person who does one job. When the peg no longer matches the hole, they are replaced or reassigned. Which is sad, because people with valuable skills are often underutilized. I feel like companies like Valve might have the right idea going forward.
The problem is that larger organizations actually need to have a defined process of some sort. There is a point beyond which individual judgment doesn't scale.
In general I believe that Google has a pretty good process. However every process has bugs. And I happen to have encountered one where they pick people who are qualified in one role into a more experimental one, and it only sometimes works out for them.
