Was the founding of the German Empire a successful nation-state?

Yes. It wasn't as successful as hoped, as there was hope for a Greater German solution, including Austria-Hungary, but the most important demands, such as the annexation of Schleswig-Holstein and Alsace-Lorraine, were met, and the German Empire, especially during its early years, was economically and militarily the strongest nation on the European mainland. Political missteps, as well as excessive militarism and nationalism, led to World War I and thus to the fall of the Empire, but I wouldn't really call this the cause of the nation's founding.