It often protects me from things I miss. E.g. when I'm TDD'in a feature and I'm implementing the 5th-6th test I often find myself inadvertently breaking one of the earlier tests.
I do think there is such a thing as overtesting - i.e. regretted tests. TDD actually protects you from this to an extent by tying each test / modification of a test to a change in code.
Runtime assertions definitely give you more bang for the buck (they are ridiculously cheap) but they are complementary to tests, not a replacement. It attacks the same problem from the bottom up instead of top down.
I also find that when you combine the two, the tests become more useful - previously passing tests will fail when they trigger those assertions.
I do think there is such a thing as overtesting - i.e. regretted tests. TDD actually protects you from this to an extent by tying each test / modification of a test to a change in code.
Runtime assertions definitely give you more bang for the buck (they are ridiculously cheap) but they are complementary to tests, not a replacement. It attacks the same problem from the bottom up instead of top down.
I also find that when you combine the two, the tests become more useful - previously passing tests will fail when they trigger those assertions.