I hand-rolled my own authentication on this site. But I had a good reason! Remember all the talk above about making things super fast by collocating the node servers and databases close to users? Well, I'd kinda undo all that work if I used an authentication service.

This is a great example of how fundamental architecture decisions limit your subsequent decisions, and it’s your job as a designer to make sure that the final result makes sense and reflects good tradeoffs, not just the state as you go. Hand-rolling auth (a real risk) so you can use this unnecessary distributed deployment scheme (no real value) is a bad decision.