It could be that Vulcan family names are extremely long and complicated, indicating multiple generations of lineage or elaborate clan interrelations, so the pronunciation difficulty is more due to the length and intricacy than due to the phonemes involved.
I wonder is a part of the difficulty of pronouncing Spock's family name might be linked to the range of frequencies Vulcans can hear?
Vulcan are depicted as being able to hear frequencies Humans can't. Humans can hear (basically) between 20 hertz and 20,000 hertz. Amanda said she could pronounce the name after years of practice.
Could Amanda have trained herself to produce sounds, that she herself could not hear?