1. The logical unit within a Beam pipeline is a transform. Either a small transform like a ParDo or a larger composite transform. Unit tests should focus on these units, rather than probing specific behaviors of the user-defined functions.
2. The way that a runner interacts with a user-defined function is not necessarily obvious. DoFnTester allows testing non-sensical cases that wouldn't arise in practice, since it allows low-level interactions with the actual UDFs.
Instead, we should encourage the use of TestPipeline with the direct runner. This allows testing a single transform (such as a ParDo running a UDF) in context. It also makes it easier to test things like side-inputs and multiple outputs, since you use the same techniques in the test as you would in a real pipeline, rather than requiring a whole new API.