``However, if you run existing code or use an "off the shelf" open source application, chances are, it will be tested and developed on MySQL/MariaDB and not on Postgres.''
That was my experience back when I was looking for web site software a few years ago. It's not so much that the "off the shelf" application hasn't been tested against PostgreSQL but it's almost certain that the developers only considered MySQL, taken advantage of non-standard SQL statements that are available in MySQL, and locked users into using only that database. I downloaded untold numbers of web site packages and found that most of them had used things like MySQL's "REPLACE" statement which meant they wouldn't be useful in my existing PostgreSQL environment without significant reworking. Standards, shmandards.
Ideally, it'd be nice if more developers would write their application to use some of the database abstraction layers that are out there (PEAR, ADOdb, etc.). At least then users would be able to merely use the database they may already have installed.