Not quite! The meaning is more like "The difficulties are in the details." E.g., while the concept of a polution-free car is easy to understand and subject to broad agreement, how to design and produce one is a huge problem full of many engineering and manufacturing challenges!
Invoking the devil, of course, makes it idiomatic. Not being a native speaker, I suspect none-the-less, that the devil may be as good a stand-in for "big trouble" in Spanish as in English. Louanna007, your literal translation may be as good as any.
For instance, whether an idea will work depends not only on the concept itself, but on how well the numerous small details are handled.
As an example, you have a great idea for a new product, but how you handle the details will determine your success -- do you have trucks to deliver the product to the stores? do you have reliable equipment to make the product? do you have any necessary permits or licenses? etc.
Although they're not the product you're trying to sell, these details can be difficult to manage, yet are crucial for your success -- the devil is in the details.