While doing the state script backport to morty, I had an issue where the version of mender-artifact I was using would error out. Notably this was when I was attempting to add the example-state-scripts recipe. In this scenario, the call to mender-artifact would try to use the '-s' option. The invocation would fail with an error message but evidently no error code. The bitbake build would complete successfully but the actual artifact files would not be generated. I tried a few invocations of mender-artifact by hand and it seems that specifying an invalid command line parameter does not cause an error return from the executable. I'm not sure if that's true for all invocations of mender-artifact but certainly with the use of an invalid command line parameter.
I assume having the executable return an error code would allow bitbake to detect the failure and bubble that back up rather than having it buried in a log file.