Do you have more specifics, and/or have you advised them of the difficulties? It seems to use the same "mach" build process as does firefox, and I build FF developer edition regularly (not every day, but damn near): https://github.com/servo/servo#normal-build

While investigating this, I have deep sympathies for whoever has to work with that taskcluster silliness because yikes that is some A++ grade obfuscation as compared to a .gitlab-ci.yml or .circleci or even the .travis.yml they migrated off of