Ironmonger is definitely a strong card, no disagreement there. A few things I think are worth mentioning:
Ironmonger-BMIronmonger is surprisingly strong all by itself -- solo play suggests that straight Ironmonger-BM is similar to Smithy-BM (14-17 turns to get 4 Provinces and a couple of Duchies). So while Ironmonger generally fits into an engine, that engine has to be fast enough to beat Ironmonger on its own, which is non-trivial.
Ironmonger in an engineIronmonger seems to me to be at its *worst* when it finds other actions. Let's consider the separate effects:
Treasure: +1 card, +1 action, +$1, potentially discards copper. That's a bit stronger than a typical $4; Donald has said that +1 card/action/$ would be a boring but balanced $4, and this adds a copper filter or information about your top card.
Victory: +2 card, +1 action, discards victory. That's a lab+, which is better than a strong $5.
Action: +1 card, +2 actions, usually will not discard your action. That's a village.
Therefore, in a vacuum, Ironmonger loses relative value as your action density increases -- because you've paid $4 for a card that is increasingly likely to be a $3 card. Now, obviously, if there are no other villages and you need one to make your engine work, maybe that's not a concern (the same way you'll buy multiple Worker's Villages even if you don't need the +buy).
Conversely, Ironmonger tends to be ok with early greening, since the more green you have, the more powerful Ironmonger is. However, I say "ok" instead of "really likes" because you have to be careful not to fall into the same trap as Crossroads -- the more green you have, the more likely Ironmonger's blind draw is another non-helpful green card!
Ironmonger and dual type cardsFollowing the same analysis as above:
Victory/Action: +2 cards, +2 actions
Victory/Treasure: +2 cards, +1 action, +$1
Both of those are strictly better than lab, so the more of those you can put in your deck, the better Ironmonger becomes.
Works with/Conflicts withIn addition to the above, Ironmonger works well with cards that care about the top card of your deck. Mystic is the obvious choice but there may be others (Lookout, perhaps).
Ironmonger does not work well with cursers, since hitting a curse reduces Ironmonger to a straight cantrip (which is worse than Vagrant, a $2). It's a mixed bag with Looters; hitting Ruins is a Village plus Ruins filter, which isn't bad for a $4 but it isn't great. More important is that in most cases you're likely better off just going for the Looter (Marauder/Cultist) or ignoring it (Death Cart, unless of course there's a Death Cart enabler on board) than comboing it with Ironmonger. That being said, Ironmonger is a strong choice if you are building an engine that ignores the available Looter attacks.
Ironmonger and opportunity costI'm going to pick on this statement:
if you're building an engine, pretty much any engine, Ironmonger is going to help all the way, and it isn't going to hurt even a little. Because it's always a cantrip it never hurts to play it.
You could say the same thing about Caravan; in fact, you *should* say the same thing about Caravan. But it's important to take opportunity cost into account as well. It never hurts to play an Ironmonger, but sometimes it hurts to *buy* an Ironmonger, if your engine could get off the ground faster if you bought a different $4.
That being said, Ironmonger tends to be a bit better than most other spammable cantrips, because early in your engine build, Ironmonger is likely to hit copper and estates, cycling your deck a bit and giving you the more powerful effects; later, as you trim your deck, set up filtering, and/or increase action density, Ironmonger gives you the +actions you need.
The basic takeaway, though, is that Ironmonger in an engine works much better with a gainer, so that you're not giving up the opportunity cost of buying a different $4.