Blubber outputs artifact instructions in the Install phase. It outputs builder instructions in the PreInstall phase. This combination means that you cannot describe a multi-stage Dockerfile using Blubber where one variant builds a collection of artifacts that are needed by a subsequent variant's build instruction.
My current use case for this is splitting up multiple builder commands to reduce layer invalidation. Variant A runs a builder to download and install a utility program that will not change often. This utility is needed in Variant B's builder to install other components.