I'm beginning to resist the Herald hype. I'm doubtful that it's a top 10 $4 card. Herald can help make engines that are lacking either +actions or +card (or both), which is powerful, of course. But it is a mediocre card on boards where you can't get thin, or there isn't good payload. I feel like that is almost ~50% of games where Herald is mediocre.
In general, I have a higher opinion of the cards that are great in all decks: Ironmonger, Jack of all Trades, and Magpie.
Herald is not a mediocre card on boards where you can't get thin. I mean, sure, you don't want it in BM or slogs, but engines have been getting more and more common over the past few expansions, and it's one of the best engine cards in the game.
Say you're running an engine without any form of trashing - they are fairly rare, but do exist, especially if there's some other form of guaranteeing consistency (for example: delayed draw, Scheme, Summon, Prince on a Colony board). In those cases, you'll probably buy an Action card every turn, and it's likely there are gainers or +Buy so you'll acquire them even faster. So after 7 or 8 turns, you probably have 10 Action cards and your Action density is 50%, and it only goes up from there.
Say you have an Action density of 50%, and assume the average thing happens with 2 Heralds. You play one Herald, hitting C/E (so it's a cantrip), and then another, hitting an Action card (so it's basically a Lost City). Vanilla Cantrip + Lost City = Village + Laboratory. So your two Heralds are as good as a Village and a Lab, which I wouldn't call mediocre. And as you add more and more Action cards, your odds of hitting one with Herald increases, meaning you get more Lost Cities and fewer vanilla cantrips.
Of course, the other extreme case is where you trash all your starting cards, in which case every Herald is a Lost City without the drawback of giving your opponent +1 Card upon buying it. That's pretty insane for $4.