In theory, but in practice ultra-nationalism is by nature very aggressive against the "non-citizens" or contrarians, which easily evolves into downright fascism.
It's antagonistic against those who completely refute the nation, which is to be expected; any ideology is antagonistic towards anything or anyone that refutes its beliefs, moreso if they otherwise share many other beliefs.
However, it's not antagonistic towards non-citizens that still live under its auspices. As example, Juan Rico's family are very rich and well-to-do and operate just fine within the nation of Earth in the novel AND the book, without ever being Citizens for several generations; it really is just a restriction of who can be politically active, when it comes down to it.
Sure there are differences and circumstances, even buffers, depending on the particular culture we are talking about, but there is a reason why this concept has such a definition as "ultra".
The primary issue is that it holds one nation above all, and thus, cannot accept other nations as superior; it doesn't care about cultures, so long as they accept the one nation precept. The Earth in Heinlein's novel was able to reach a global nationhood and still have many different local cultures still because of that. Without that unity, it definitely has more demerits than merits, honestly.
Nationalism is good, a country should have a strong and stable image of itself, otherwise, its culture, however many it has, can be in danger and its stability, both economical social, and political too, but when a country or group enters the stage of ultra-nationalism... they already stopped being in a healthy state.
Personally, I agree, so long as there are competing nations. I don't see Earth in reality ever doing away with competing nations, at the very least not within at least double digit more generations, so I really don't see any benefit to adopting any such ideology, especially since such would likely be rooted in core concepts of xenophobia and mystical right to supremacy (read: pseudohistorical BS) rather than uplifting one's nation.
If we were to, somehow, though, and get to a place similar to Heinlein's Earth, where it is one nation and one global culture but many local cultures all supportive of that single nation and global culture, I'd argue that it could actually be a considerable benefit, so long as the system's corruption can be managed to a better degree than all current government's corruption, considering that worldwide logistics are the number one issue holding back most people (we've had more than enough food for everyone to eat pleasantly whenever they want for some time now, but the logistics of getting that food to the people who need it has been, by and large, the greatest issue; something a single government could create a department for that then distributes food on a global scale would be daunting, initially, but certainly doable). It would have to remain non-fascist and non-corporatist without delving into national socialism or communism, either, riding the razor's edge, so any such would probably break down once a sufficiently charismatic person was able to put themselves into a position of power and thus wrest control of the system from its rightful form; functionally, as vaporware as Marx's Utopia.
You are right... by itself, it is neutral, and both sides can become ultra-nationalist and even fascists, because contrary to what people think... fascism isn't a thing only of the "right-wing", there is no shortage of "left-wing" fascists in history...
No, you can't get "left-wing fascists"; By definition a fascist cannot be left-wing, due to certain irreconcilable beliefs that that ideology holds; however, you can have national socialism, which effectively functions as the left-wing equivalent.
and the things that have been done for... "the good of the country" are... bone chilling, both to "non-citizens" and citizens.
Very much so. Similarly, the number of things that have been done while falsely holding up "the good of the country" while the true reasoning behind it is quietly shelved by the masterminds, out of sight of the public.
