It seems to me, first of all, that there's a threshold of software complexity, beneath which writing tests is a net loss in time and productivity. If you have (as I often do) a couple of hundred LOC spread across a few files, with one or two models/views, then there is negligible gain to automated testing - by the time you've set up/torn down the test DB, you could have refreshed the page in a browser 5 times and seen all the error messages in there. I accept that this threshold can be hit very rapidly once you get towards a modest website, even; but my day job basically involves writing toy SPAs with a very simple API behind the scenes, built and then left chugging along until they're out of date, so I often sail under it.
A bigger anxiety I have about TDD is that, while there are no end of resources on using tests to drive software design, I haven't come across much about good design of tests (suggestions very much welcome). I rarely have confidence that my tests are any more use than assert_true(true). You test for the constraints you anticipate when you write the tests. I don't trust my code-fu that far, and so my tests inherit their author's impostor syndrome.