Advantage: the ingroup is not a matter of debate.
Readers of this blog know that I am highly critical of ethnonationalism serving as the core of nationalist activism. In contrast, I support a primary emphasis on pan-European racial nationalism (or "White nationalism"), with ethnonationalism serving as a secondary, lower-level emphasis within that broader racial nationalism.
However, I do admit that ethnonationalism has one distinct advantage over racial nationalism as it is practiced by the pathetic, dysfunctional "movement" - ethnonationalists have the advantage of a "ready made" and well defined ingroup, one that is not a matter of debate and endless nitpicking analysis. Thus, ethnonationalism side-steps all the sterile "debates" that typify the "movement" (e.g., "who is White?") and more rapidly achieves pragmatic political objectives.
Thus, to a Hungarian ethnonationalist, it is clear that the ingroup is ethnic Hungarians. It does not matter whether these are dolichocephalic or brachycephalic, whether they be Nordic or Alpine or Mediterranean or Dinaric, whether or not a given Hungarian has an "admixture" percentage of 2.1345784562% East Asian or not, or what are the eye and hair colors of Hungarians, etc. They are Hungarians, part of the historic Hungarian nation, and that is sufficient. Certainly, cultural and other issues influence things as well as ethnic identification: a Muslim Hungarian may be rejected as well as one who has married, say, a Negro. But that is part of the grand definition: a person who is ethnically and culturally Hungarian is considered a member of the Hungarian nation - and that's it.
Racial nationalists - at least those who claim to be pan-European - could in theory do the same thing. They can side-step all the nitpicking that bedevils the "movement" and proclaim their ingroup to be those who are ethnically and culturally European, which is - or should be to any reasonable person - as obvious as saying that someone is ethnically and culturally Hungarian. Just as a Hungarian ethnonationalist would (or should) reject intrusive elements like Jews, Gypsies, and Turks, so could (and should) the European nationalist.
Thus, the advantage of the ethnonationalist is merely a relative one, due to the deficiencies of racial nationalists. These deficiencies could be corrected, given the will to do so, which means recreating a real Movement on the ashes of the failed "movement."
I won't expect that to happen any time soon, however.