That's a great question. Unfortunately, I can't answer it. Because my father was a linguistic scholar, and I was exposed to all that growing up, I don't have the average perspective you're looking for. FWIW, since I'm not actually a linguist myself, I'm aware of pronunciation patterns mostly on a subconscious level. But I do know some stuff from dad. For instance, about that Orion example I mentioned above: As a word of Greek origin, OR-ee-on makes sense as its pre-Great Vowel Shift pronunciation. But, since we generally honor the shift, we say or-RYE-on. In the case of that word, knowing how to pronounce it correctly is strictly a function of knowing that it is a word in colloquial use. Since it is, the Great Vowel Shift applies (in that case). But there are nuances and exceptions, and semi- provides an example of that. The prefix semi- is Latin (from Greek hemi-), so pre-Great Vowel Shift semi rhymes with me. Don't ask me why, but Americans pronounce it that way when they say semiconductor. Perhaps the reason for that is that it just roles off the tongue easier that way (it's a five syllable word, after all), or maybe it's because it's (originally) a non-colloquial technical word; or maybe it's a combination, I don't know. On the other hand, the word semi by itself (meaning semitrailer), requires rhyming with eye, and it is definitely a word in colloquial use. If you say it the other way, it will sound ridiculously pretentious. I personally suspect that that's because the American ear can tell that rhyming semi with me harkens back to the way Europeans might prefer it to be pronounced. As far as I can tell, native American speakers have the instinct to pick that up more or less automatically, and reject rhyming semi as a word by itself with me under any circumstances. All that's my non-expert view.