Operational complexity at scale and prior experience with it. It is one thing to run one instance system and completely different story to run a clustered setup

Operational complexity at scale and prior experience with it. It is one thing to run one instance system and completely different story to run a clustered setup for large amounts of data with performance consideration. MySQL also proved very good at scale, at Facebook, YouTube, Uber, etc. and there are a lot of people with experience running it. Uber wrote a very good article about their switch, for them it was more about performance, though a little bit controversial.