Jenkins does not recognize ReadyApi maven plug-in test failures as unstable build
You are ignoring test failures in your plugin config in this line:
Actually, that's not quite correct, according to the documentation, this property determines if a test will continue running if it encounters an error:
Continues the test run even if an assertion triggers. Possible values: true, false.
After your test runs, you can use the JUnit plugin to parse the results, and that will pass/fail the build.