Good point about Nassau and Limburg-Stirum case...Limburg-Stirums were one of the oldest families and also a senior branch of at that time extinct Dukes of Jülich,Kleve und Berg...but Countess Mathilde was product of a non-equal union between her "equal" father Count Leopold von Limburg-Stirum and a "commoner" Julie Marie Barre...so,despite the fact that she has descended several times from Wihlem von Nassau or known in Dutch history as Willem de Rijke and also from Ursula,sister of Amalia von Solms she was not treated equal...
There is also an example of Countess von Harrach and Friedrich Wilhelm III...She belonged to mediatized House of Harrach but was not treated equal...the official reason she was not treated equal was because she was a member of collateral branch of the family who did not gain those equal privileges as the main branch did...that's why she was ceated Princess von Liegnitz!
Here is what Prince Nikolai Romanov said about possible marriage of "non-equal" Princess Obolensky and "equal" Countess Harrach with the Grand Duke for example:
"Russia, with its very Germanic notion of dynastic propriety, found itself accepting all the Almanach de Gotha rulings.
And so if some unfortunate Russian Grand Duke wanted to marry a Princess Obolensky, descendant of the Grand Dukes of Kiev, who reigned in Russia, at the time his Romanov ancestors were probably still lurking in the woods, draped in pelts or wading through the marshes of East Prussia or Pomerania, he would have had to change his plans.
That marriage would have been impossible, but an Austrian lady, say a daughter of an Illustrious Highness, Count von Harrach zu Rohrau und Thannhausen, lord of the county of Rohrau, Freiherr zu Prugg und Pürrhenstein, lord of Starkenbach, Jilenice, Sadowa & Storckow, would have been acceptable!"
So,for a Grand Duke Countess von Harrach would have been in some sort of way acceptable,unlike Princess Obolensky!
There is similar example with Wilhelm I and Princess Elize Radziwill...although from an equal family she wasn't considered equal because it was discovered that Radziwill family bought the title of Reichsfürst from the Emperor rather than earned it!