In a Multijob setup, we create and archive some artifacts in a sub job, which we afterwards copy into the parent job to be consumed during a "docker build" step (which directly follows the "copy artifacts" step).
Before switching to S3 artifact manager, everything went fine, but afterwards the jobs started to fail because the "docker build" step couldn't find the artifacts Jenkins said it had copied successfully.
We could reproduce this in a test job. There is some delay between "artifacts copied" and "artifacts being accessible", and the artifact in our case was just 15kB.
This renders the plugin unusable (for us), hence the "Blocker".