Prototype only what you need to validate your ideas in a very short time; hammer out a realistic prototype, a facade of the experience you have envisioned in the sketch phase. Design a barest minimum but usable prototype, taking advantage for instance of of Pop App, an app that transforms pictures of a story board into clickable UI; think of your prototype as an experiment in order to test out hypothesis;
Another important criterion is the expertise of the trainers, both into the subject as well as in training and facilitating teams and individuals. As mentioned before, some providers have build experience by applying the framework themselves, while building digital products. Others have a background as trainers (for example in Agile, Scrum or Design Thinking) and added Design Sprint training to their curriculum.
But probably the most valuable benefit of design sprints is that they introduce stakeholders to the importance of validating ideas with real users. Google has orientated the whole week around building a prototype that users find easy to use. That is a valuable lesson for colleagues who can often be more focused on their own agenda, rather than that of the user.