I would call Harvest the weakest card in the game, its biggest virtue is that it generates cash, which is always on the board. Mute, at least, has the virtue of being a trasher (e.g. can be used as the only option in a Familiar game to get rid of curses) and a VP gainer which are things that, rarely, can be worth spending a turn to buy if there is no other option on the board and also gets a lot less bad when you are already buying a pot for something like Familiar. Pstone is another very weak card, but it at least can be strong on 3 or 4 player games with lots of junking (e.g. Mountebank, Ambassador) and actually quite good with colonies on those boards.
Where Harvest actually gets useful is where it does something better than two silvers. Notably, it can be the only +$ option for a Lib, Owl, or other limited draw engine. These can be exceptionally powerful with strong attacks or alt-VP (e.g. Temple) allowing you to build for a long time. The discarding can also be helpful in its own right, for instance rare niches can include using it as a B-crat/Fortune teller/Rabble counter or activating Tunnels; in both cases it is normally too weak to bother but can get a lot a stronger with favorable setups (e.g. having a Squire/Lib engine against Rabble, using it with a Remodel/Courtyard/Village engine for enabling Tunnels).
I do think it might have a legitimate synergy with Village green. Search space is the same size as anything that discards from hand at start (e.g. Count), can discard multiples per play, and generates a respectable cash load while discarding. I have definitely used it as the only discarder in the game to good effect.
Ultimately though, a best case scenario for Harvest is discarding 4 cards for $4. This costs you one draw slot and one action for using up a single $5 gain. You can get the same $4 for no action, two draw slots and two $3 gains. On most boards actions are more valuable than draw slots and a single $5 gain is much more valuable than two $3s. Unless there is nothing else at $5 or I really need some specific function like discussed above, Harvest will only be a viable shot when +actions are not limiting and where I need the unreliable cash now more than whatever else is on sale at $5.