Don't know the exact details but they mentioned is full of operational footguns, like using it slightly wrong can utterly tank your performance or availability. Many operations are unpredictable so is hard to know what to expect.

Another reason they mentioned is InnoDB, in particular that is the most battle-tested and well-engineered storage engine. With enormous amounts of engineering effort in hardening it for performance and for stability and operability at scale by companies like Google and Facebook.

They also like Vitess.

Pg transactions seem more likely to deadlock compared to MySQL, IME. Its replication story was also worse for many years, though it's catching up.

Pg connection limits are also lower because of resource allocation. And lack of query hints can really hurt when Pg's planner goes sideways.

That said InnoDB's online DDL lagged Pg for quite a while. Generally feels like they're both getting better and more alike all the time.