I second your observation. Authority sites can rank for almost any term, especially at the beginning of publishing of new content. Then that traffic gradually fade and dissipates to other sites serving similar contents.
Super targetted sub niches do take a long time to gain momentum. But once traffic is established, it is hugely more reliable than an all encompassing authority in my view. I also observe that the momentum does take 3 month~6 month to establish. At the beginning the traffic volume is dismal at best. The risk is that sub niches can only get so popular and it's almost impossible to rank and gain traffic for other terms outside of the niche.
My general business strategy at the moment is to build both authority sites as well as sub niches interchangeably to "lower" my risk. I observe that my niche sites with good content are clawing and winning traffic from other "spam" authorities that offer no value, that are pretty much ranking with blank pages with keywords and shallow to nonexistent content.