As of 0.5.5 (haven't tested against earlier 0.5.x releases), this is once again throwing during build:
[broccoli-persistent-filter:Babel > [Babel: @embroider/macros]: Babel: @embroider/macros] was configured to throwUnlessParallelizable and was unable to parallelize a plugin.
Same issue as #33, but apparently now for @embroider/macros instead. May be a broader issue with @embroider/macros and not specific to this repo?