Just posting to point out that a $4 cost card that's usually "only a Lost City with an auto play clause" is a really good card. Lost City costs $5 and has a significant on gain penalty.
Sure, I am not saying that lost city overall is strictly better or anything.
But it seems to me that herald is just a 'not always functioning' lost city when you have it. If you reveal an action, you can play it. But so does lost city: if you draw an action, you always have an action to play it. if you don't reveal an action, it's just a cantrip with herald. In lost cities case, you have the additional card in your hand, which is a-ok. So, sure, pricing and on gain effects matter for the total evaluation of the card and all, but the actual on play effect, herald is just a pretty bad lost city. Or am I missing something?