The last advantage of unit tests is that they are faster. I have written enough integration tests to know, that they can be nearly as fast as the unit tests.
This is the key point, and not only "the last advantage". When the project gets bigger and bigger, your integration acceptance tests are becoming slower and slower. And here, I mean so slow that you are going to stop executing them.
Of course, unit tests are becoming slower as well, but they are still more than order of magnitude faster. For example, in my previous project (c++, some 600 kLOC, 4000 unit tests and 200 integration tests), it took about one minute to execute all and more then 15 to execute integration tests. To build and execute unit tests for the part being changed, would take less then 30 seconds on average. When you can do it so fast, you'll want to do it all the time.
Just to make it clear: I do not say not to add integration and acceptance tests, but it looks like you did TDD/BDD in a wrong way.
Unit test are also good for the design.
Yes, designing with testability in mind will make the design better.
The problem was that over one half of the development time as for writing / refactor tests. So in the end I did not want to implement any more features because I would need to refactor and write to many test.
Well, when requirements change, you do have to change the code. I would tell you didn't finish your work if you didn't write unit tests. But this doesn't mean you should have 100% coverage with unit tests - that is not the goal. Some things (like GUI, or accessing a file, ...) are not even meant to be unit tested.
The result of this is better code quality, and another layer of testing. I would say it is worth it.
We also had several 1000s acceptance tests, and it would take whole week to execute all.
