Yes the Dutch Royals used to be prince(ss) of Orange-Nassau as well but since Queen Beatrix things have changed. Only the daughters of the King have the title next to their other titles.
The nephew and nieces of the king are count(ess) Van Orange-Nassau van Amsberg or count(ess) van Orange-Nassau, jonkheer/jonkvrouwe van Amsberg.
The daughters of Guillaume IV could not be styled as princesses of Parma as they were all Grandducal Highnesses princesses of Luxembourg and princesses of Nassau-Weilburg.
Grand Duchess Charlotte married her maternal first cousin prince Felix de Bourbon-Parma. He was incorporated into the Luxembourg nobility as HRH prince de Bourbon de Parme. As such all their children had under Luxembourg law the right to that noble title after their dynastic titles.
The same applies to the male line descendants of Felix.
That might very well be the reason that Henri elevated the children of his brother Jean and cousin Robertfrom count(ess) to prince(ss) of Nassau because they might otherwise claim the higher noble title. Outside the main branch the junior branches don't use their Parma title.
Continuing with the thread on in-house unions: do, in fact, the Belgian royals in male-line descent from King Leopold I have multiple titles? Is Elisabeth a princess of Saxe-Coburg and Gotha, in addition to being the crown princess of Belgium and the Duchess of Brabant? If so, then perhaps it would count as one of those situations of unused titles (which I previously brought up): one might be A prince(ss) of a country X, but never called that.
Certainly nobody would ever call, right in Her Majesty's hearing, Queen Elizabeth II of Great Britain as a princess of Saxe-Coburg and Gotha ...
One presumes that people would be less hesitant about referring to her children as princes and princess of Denmark ...
Now to be sure, the Greek royals from King George I on have always styled themselves a princes and princesses of Denmark; but not the Norwegian royals. Swedish royals were also princes and princesses of Norway, until the person union between the two countries ended in 1905.
The Dutch royals are also princes and princesses of Orange-Nassau -- correct? But members of the grand duchy of Luxembourg, although princes and princesses of Nassau-Weilburg until the deaths of all six daughters of Grand Duke Guillaume IV, are not styled as princes and princesses of Bourbon-Parma ... Or are they? I heard that they were, until the marriage in 1981 of Grand Duke Henri (then heir to the throne) to Maria Teresa Mestre. The then-head of the house of Bourbon-Parma did not approve the marriage. So Grand Duke Jean severed all ties to this dynasty. Is this correct?
As for some of the non-reigning houses: what about the Bulgarian royals? Have they been styled as princes and princesses of Saxe-Coburg and Gotha? I know that the children of Queen Maria II of Portugal were, in addition to being Infantes and Infantas of Portugal, princes and princesses of Coburg, as well as dukes and duchesses of Saxony.
Matters are interesting in Spain and Portugal, which have always honored the maternal heritage: one can also be an Infante/Infanta of Spain through his mother. Such was the case with the children of Maria de las Mercedes (elder daughter of King Alfonso II of Spain), who were also princes of princess of Bourbon-Two Sicilies. Similarly, the children of her younger sister (Maria Teresa) were Infantes and Infantas of Spain, as well as princes and princesses of Bavaria. The latter titles, of course, were forfeited when their father (Prince Ferdinand of Bavaria) renounced in 1914 his Bavarian royal titles and succession rights to the kingdom.
But as for the Romanian royals, I'm not sure if they've been styled as princes and princesses of Hohenzollern-Sigmaringen ... I have no doubt that that's what they were/are; but I'm not sure if they've actually used these titles.
Since the house name of the Brazilian dynasty got hyphenated to Orléans-Braganza, it goes without saying that the princes and princesses of Brazil (i.e. those descended from Isabel) have also been princes and princesses of Orléans -- correct? Or perhaps I'm mistaken, and they haven't used their French royal titles. Perhaps calling oneself a Prince(ss) of Orléans-Braganza is a way of simplifying things ...