My company recently looked at MySQL vs PostgreSQL. While we chose Postgres there is one thing that stood out in my research, I haven't found any comments on this:

We were looking at scalable, HA, possibly multi-DC workloads and such, and yes, relational DBs are not necessarily the right choice, but MySQL looked a bit better in this regard, especially for master-master replication. There are several high-profile teams that have huge production deployments running on MySQL (IIRC Uber and Github) and there are a lot of things to be learned from them.