Heavy Duty boots massively boosted fire types, which was huge. Cinderace in particular annihilates it. Add it just got crushed in OU by Mandibuzz, Hippowdon, and Urshifu. Even down in UU G-Moltres beats it solidly.
Yep. Aegislash was Uber in Gen 6 and 7 for a fair number of reasons, but tiering isn't a be all, end all of power.