As I said, the Duration effect lets you play it twice, and the Chancellor effect speeds up one of the shuffles, so you're playing it more often than something like Moneylender. For a normal action card to do the same thing, it would Chancellor and topdeck itself 50% of the time you play it, and be a cantrip the other 50% of the time! But that's fine, probably it needs that boost. Also, because it doesn't make your deck smaller, it doesn't accelerate itself - the more you trash, the more often you see your Steward.
As for how it performs later in your engine, it's probably worse than Squire, which can also give +$1 +2 Buys. If you draw your deck, the Duration effect (and the Chancellor effect) are totally irrelevant because you can play it every turn anyway, like Merchant Ship just becomes a terminal Silver. OK, maybe you can get rid of some leftover Coppers, but you can't have that many of them by now, and Squire does so many other things, I don't remember the last time Squire was just my engine's +buy.
With Chapel or Steward, I always want to get my deck small enough that I can play everything every turn.
It's not a bad card, but it doesn't play at all like you think it does. Probably it's best in slogs, Gardens for the +4 buys, Vineyard to get rid of a Copper when you get your Vineyards (+buy is also important), or junking games without strong trashing, in which Copper is just a lot better than Curse or Ruins. Probably it can build an engine, but nowhere near as fast as Steward or Remake.
