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.
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.
When I was there they hired people with one background and hoped they would be capable of the other. This didn't work out for about half of them. Making things worse, they had no procedure to say, "OK, we hired someone we think is good, but this is clearly the wrong role for them." Which lead to losing a lot of people who had potential.
Making it personal, I interviewed for a pure dev position then was offered a job as an SRE. It turns out that I don't make a good sysadmin. After I left, I was amazed at how many people I met knew of someone else who had had the same experience.
Ironically recruiters see a year as a Google SRE on my resume and think I might want to join SRE teams at other companies. Most of that stopped after I changed my Linked In profile to say that I am never interested in SRE jobs.