One big factor that keep us on MySQL is the MyRocks engine. We have huge databases with billions of rows. The MyRocks enable the use of it with heavy compression, that PostgreSQL can´t handle it, as it is much slower and uses 30x more disk usage, even with heavy TOAST tuning and/or ZFS compression.
To be fair, at the scale of your use case there I really hope you have a proper DBA who understands multiple database systems and their details, and is able to make the best choice for your setup. (At some point the commercial and/or oddball SQL servers become an option too…)
For everyone else who's in most cases not even stuffing a million rows into their database… just stick with Postgres :)
