[koide]

a) Premature optimization b) Implementation mistake

Using natural keys may offer better performance in certain cases, but does not protect you from b, as you might select a wrong key, or even forget about putting a PK on the table, for example.

Surrogates help you manage the ever changing and interconnected world: the immutable name of something that has been shared between systems is no longer immutable? This type of scenario is also very common and can cost thousands as well.

Which type of problem is more likely in your application? That's up to you to decide.