Well that is the debate exactly. On the one hand nationalism lead countries such as Germany into unification and even dominance on the European continent, on the other hand it evolved into odd ideologies like fascism in Italy.

Also, what exactly do you mean by nationalism? Do you mean nationalism as an (political) ideology, as for example in the greek rebellion of 1821 (not sure about the year), or as a political tool the way Bismarck realpolitiked the shit out of it? Do you mean the philosophical ideas like the way Herder actually introduced the term, or the social stratification as it is implied in many other socialogists/philosofists. I am not really asking these questions btw, i just wanted to show that nationalism is a very difficult term to work with, let alone giving it qualitative values like succes or failure.

A failure in the short term and a success in the long term.

It was a failure in that it gave rise to hypernationalism and the associated ethnic elitism of the early 20th century. Empires, like the Ottomans, or Babylonians, were inclusive and could be tolerant of multiculturalism, because they did not assemble around a specific geographic region. Hypothetically, anyone who submitted could be an Ottoman, even nonMuslims and certainly nonTurks, although they may not have had equal status. The growth of nationalism, and away from empire, centered the political unit around a single geographic region, often with one ethnicity.

But after horrible colonial and ideologic wars of the late 19th/early 20th century the nation state gives rise to stable political units. Empires are ever expanding, but most countries have no territorial ambitions. The Crimean takeover is an exception that proves the rule. No one one would have questioned the might-makes-right principal in the 17th century. But now we are outraged. Exactly because the principal of Nation-State gives rise to the expectation of stable borders.

Every state currently existing in Europe justifies its existence through nationalism - that it is the state that represents the national identity of the people within its borders. Ergo, I think we can say that nationalism was a massive success.