I think you are getting lost in the details. Databases are inherently complex and there are sharp edges as well as tricks you need to learn for each one. Beyond that row stores like MySQL and PostgreSQL are essentially interchangeable. The biggest technical success factor is whether you understand the DBMS well enough to use it effectively. The biggest business success factor is licensing: MySQL is GPLv2 and PostgreSQL is BSD.

Specific features are somewhat interesting but at this point not many teams are starting from scratch on MySQL or PostgreSQL. It's generally the case you already have experience on one or the other or both.