Every card has both light and dark, positive and negative potential outcomes associated with its core ideas...based on choices made and so on. That's why I don't take reversed (upside-down) cards into much consideration in doing a reading. It might point toward one aspect of a card or the other, but it doesn't totally change the meaning, as some older reading styles put it. I think it is wise to look at all aspects of a card in every reading, to highlight the choices we have, and the different options available to us.
So what is the down side of harvest season?
The down side is more to do with the abstract notion of "you reap what you sow" more than the literal idea of harvest, fall and so on. There can be a great deal of dismissive judgmentalism in that phrase, akin to some of the shades of meaning you see in major arcana cards like "Judgement" or "The Hierophant". It can be taken, by some, as "you deserve what you got because you reap what you sow". Even more subtle is the idea of greed. If you do something "good" with the expectation of good in return...if you sow only with "harvest" in mind...is that as moral as acts of genuine altruism? Philosophers have been going in circles about that one for ages. I kind of look at it this way...what you do sows the seeds of both the act and the motive. You might sow mostly wheat, but if you throw a few thistle seeds in there too, both will grow. Authenticity and being honest with yourself is better, but that is another card, another day.
The up side to the 7 of coins is the hope and trust it engenders. It hints at hope and trust life requires. Planting seeds are necessary to our survival. It is a leap of faith (in both nature and science) that the seeds will grow, harvest will come and life will go on. The "reap what you sow" paradigm shows us how interconnected we are to Nature, the Cosmos, and to each other. This card can be taken, too, as a promise. If we put good things out, share the best of ourselves, there is promise that we will go in a good direction and get a good result.
This card is a reminder to celebrate the harvest..whether the harvest is a hard won life-lesson, or the result of well-placed hope and trust. This card is a reminder to celebrate life.