There is indeed no guarantee. But that is true with every business. I ran a small business for several years before it collapsed due to the economic downturn in 08. When I started it, I was putting in a lot of time and money with absolutely no guarantee that I would make it back from the products I was selling. is why you need to be careful in ensuring you have a proper price point relative to the demand.
One of the problems I have noticed with the Marketplace is that Epic has not agreed to share analytics data on sales. means it becomes very difficult for content creators to more appropriately understand consumer demand.