Bear in mind that my first experiences with MySQL are 20 years old now, so I freely admit that some of my disdain stems from holding a grudge both toward MySQL AB, the PHP dev team, and hundreds of bad tutorials leading to SQL injection attacks.

I still remember vividly when this was 100% relevant. https://sql-info.de/mysql/gotchas.html

Over the years, MySQL devs claimed that foreign keys were mostly superfluous, silently truncating input without warning was normal, implicit lossy data type conversions were acceptable, and quoting numeric values was fine. Don't get me started on weird default character sets or the variously confusing multibyte UTF-8 choices and interactions.

MySQL would parse CHECK constraints but not actually enforce them until very recently (version 8 point something). Let me repeat: MySQL would accept the column syntax

  CHECK (foo BETWEEN 1 AND 10)

but would happily accept -306 without so much as a warning, assuming your code was even checking warnings. Most database engines consider saving/retrieving your data to be a strict contract of sorts. MySQL YOLOs it far too often for my taste.

Even now, DDL operations are not transaction safe. Got two CREATE TABLE statements, three ALTER TABLE statements, and a CREATE INDEX? Maybe you have an INSERT right after a CREATE TABLE because it's a lookup table. If any part fails due to whatever reason, you are now in an intermediate state. One table and a new column might be there, but not the rest. How do you roll back from that? You don't. You figure out where it failed and try to re-run the commands that didn't go through. Hopefully it didn't bork a running app.

But here's a laundry list of features MySQL does(n't) support. Imagine trying to learn English but the teacher uses a regional patois and limits you to a small subset of a normal conversational vocabulary. You wouldn't know any different as a learner until you met someone who spoke one of the major dialects like General American, Received Pronunciation, or Australian English. Even though the latter three vary quite a bit, it is a far cry from the dialect spoken like this: https://youtu.be/0pBOLdZZT6s?t=120

To get a sense of what I'm talking about, compare Oracle, MS SQL, Postgres, and even SQLite to what MySQL and MariaDB have to offer. https://www.sql-workbench.eu/dbms_comparison.html

If you've got a particular problem that MySQL happens to solve, good on you. Solving the problem at hand is the most important thing. But I wouldn't count on it as your go-to option. Unless you've got a good and specific reason to use it, I'd steer clear.