In my learning, I've definitely found that holdover words with "X" are the strangest relative to what is taught. "Oaxaca" has at least two different pronunciations depending on where you are in Mexico, since the pronunciation has evolved over time. See also: xerez.
Not the best at Spanish, but it always seems to me that all "x"s drift based on location, not just a few, making it an accent for me rather than an inconsistency. "ll" is the same way.
> Not the best at Spanish, but it always seems to me that all "x"s drift based on location, not just a few, making it an accent for me rather than an inconsistency
Doesn't seem that way to me; the native speakers I’ve known tend to pronounce the X’s different ways in:
I'm not as familiar with American place names, but there's an obvious reason why those would have pronunciations that couldn't be spelled easily in Spanish.
"X" in Mexican might be shorthand for sharp mystery consonant.
In each of those, the “x” originates from Nahuatl where it had a sound of (or close to) English “sh”, which is also the sound “x” had (maybe still has, though I’m pretty sure there has been shift in Iberian accents/dialects since then) in certain Iberian regional accents of Spanish at the time, which is how “x” got into the Spanish spelling; it wasn't a stand-in for a mystery consonant.
Your comment is correct. As far as I know, "x" still sounds /ʃ/ in Galician, Basque and Catalan. In Spanish, it became /x/ centuries ago, and the spelling changed to J/G. Nowadays you only see the X in old-fashioned spellings like Quixote, Xerez o Ximenez.
The only glaring exception I run into in Spanish is "que" being pronounced "ke."