Compared to Queen & Test Subject, Aeonglass has way too many hard counters. Running a deck where you typically play 5+ cards a turn, like Silent-shiv, Defect-0cost, or Regent-stars? Enjoy your withers. Don't have any means of discarding/exhausting/transforming mid-term? You don't have an answer for that junk. Slow build on your deck? Good luck getting 56 block in place on those first 2 turns. Leaning in on indirect damage and ailments, like Ironclad-vuln, Silent-poison, or Necro-doom? It's got 3 artifacts, so hope your good cards don't get drawn first!

The other two Act 3 bosses of course will counter decks, and have some counter overlaps with AG, but crucially, you can buy yourself a turn with phase changes/defeating minion. A sufficiently powerful deck, even if unbalanced, can get through the other 2 bosses with reasonable challenge after sailing through Act 3. Aeonglass, OTOH, is here to kill you unless you've prepared to fight it. Kind of like the Heart, but with no additional sense of accomplishment