The problem with automated tests is that they only test for the narrow slice of things you actually think to test for. They don’t cover the gamut of things you didn’t think to test for.

They also only test how you write them to test for, which means if you make a bad assumption somewhere along the way your tests can’t help you find it.

Indeed, which is why it's an ever evolving suite of tests, as and when you come across problems and things that were missed, you add automated tests for them.

It's not magic, you only get out what you put in, but it is automated, which means that if you do a reasonable job you have a lot less to worry about from that particular issue in the future and now you have a much quicker way of checking for it.

Peer reviews cover two very important things:

1. Knowledge sharing and de-siloing
2. Logic and assumption checking

A fresh set of eyes and a different perspective is just so important to writing robust, quality code.

Also agreed, and automated testing is a way to partially formalise that knowledge into something that can be checked quickly and deterministically (if you are doing it right).

As i said before it's not magic and it's not a replacement, it's more of an augmentation to relieve some of the cognitive burden.

As with any other approach, it also has it's downsides, there are ways to go about it that can be actively detrimental.

In my experience a well done (and maintained) automated suite is a boon to ongoing development.