The script of the first Astérix movie combined elements from a few different books and it wasn't very successful. Then came the second, which was based in a single book. This was by far the best Astérix movie. The third one was based on one of the books but had a lot of extra stuff thrown in there, and it resulted in a resounding failure. What conclusion can be drawn from this? That you should just trust Goscinny, who was a great writer, and keep your film as close as possible to his material. With this "Lucky Luke" film they picked characters and plot elements from a dozen different books, and the resulting screenplay was a huge mess.
I still enjoyed it, but I think it could have been much better.