It's been a long time since I played so I don't know if this is still relevant, but they had a gold play that gave 20 points initially and could push like 18 points into the future rounds, not to mention they have some pretty high value bronzes that were giving 14+ points a turn. It's not that is was openly broken it just had a very strong match up into decks that need long rounds and big setups to achieve those same values.
There's the triangle of greed, and most people in the combo corner are calling this OP because it often destroys them. Aggro people should normally beat Alchemy. It's also so common you can tech against it.