This reminds me of the test suite for one of my projects where E2E tests only covered relatively simple scenarios because of subpar WS mocking support, leaving more important, complex interactions to manual (can't run in CI) or fully integrated (expensive to author and run often) testing. The situation changed only after we wrote a custom WS mocking layer over the HTTP mocking the framework provides, yielding a dramatic increase in coverage. Out of dozens of developers I interviewed, only a few solved this issue to some degree. Clearly, mainstream testing frameworks provide insufficient support for the use case.