But, what often happens is that people work very hard at creating their product. And, in the beginning, they have enormous enthusiasm.
However, by the time they get it done, they are mentally exhausted and stressed out. So their marketing -- which is such a critical element -- often becomes little more than an afterthought.
The point is, you need to pace yourself (or outsource some of the work) so you can complete BOTH tasks -- product creation AND marketing.
So that's one way things can go wrong.
But the pendulum swings the other way, too...
I've had people come to me to have a sales letter written. When I ask to see the product, they'll tell me it isn't done.
In other words, they want me to write an effective sales letter without being able to dig through the product and pull out its best benefits. This is like showing someone an invisible object and asking them to describe it.
The lesson is, if you want your best chance of success, ideally, you should have at least a "draft" copy or working demo ready. That way, your copywriter can do their job more effectively.