Quite some time ago I thought Harvest is really underrated and you should use it so much more than now, but I'm convinced now that Harvest IS quite a mediocre to bad card, but still it's rarely damaging bad. The thing is, other cards like Saboteur have boards where they shine, whereas Harvest does this not (or veery rarely) . The problem of Harvest is that is in most cases it's like Gold, sometimes better and often weaker, but Gold is always available and therefor competes with Harvest. It's like a rule that says if Thief is in play, add Noble Brigand to the Kingdom. Thief might be bought on some board, but if Brigand is always there, too, Thief becomes even less bought. So, if there wouldn't be a Gold pile every game, Harvest would be a much better card.
There are two problems with cards that are quite bad. Firstly, you desperately want to find a use for them in every game they appear (this is often true for me) and you will ergo lose a lot of time or at least notice that this card didn't do much for your win. But even worse is the second thing: You won't care about this card after playing it a few times and losing with it. You won't try out if this card is good enough in various situations because it's an uncalculated risk. You just don't know the strenghts of it.
Getting back to Harvest,
here's a game where I'm convinced that Harvest was a way better choice than Gold, because there are cards that care about actions: Throne Room and Herald, so the coin maximum of 1 Harvest increases significantly, comparing to Gold, and it isn't a stop card anymore.
Another good example where you want to prefer Harvest to Gold are Minstrel engines, maybe also Draw-to-X strategies.