The rules are similar to prove's, passing test gets one line, diagnostics messages and failed tests are still printed.
Yath output includes the job the output came from, useful when things run concurrently.
Failing tests are completely verbose.
Output is buffered until the test file is complete, so no muxing.
You can write your own renderers for yath to completely control the output.
The default renderer is called Test2::Harness::Renderer::Formatter, which lets you use any Test2 formatter as your output.
Writing a custom renderer is intended to be simple/trivial, but in practice there is still some work to do on that.
Renderers being developed or released by others (IE not me):