As far as I can tell based on recordings of languages such as Spanish and Hebrew, the phonemes /e i o u/ or /ɛ ɪ ɔ ʊ/ tend to "slip" freely between being [e i o u] and being [ɛ ɪ ɔ ʊ]. Is this true, and, if so, is it universal among languages that don't distinguish the two phoneme sets? I would think yes for the first question, no for the second.
It's certainly true that there is no phonemic contrast between /e i o u/ and /ɛ ɪ ɔ ʊ/ in Spanish.
I'm less familiar with the phonology of Hebrew, but the variety described by the Wikipedia article "Modern Hebrew phonology" also has no contrast between /e i o u/ and /ɛ ɪ ɔ ʊ/.
The Wikipedia article on Spanish phonology suggests certain conditions for /e i o u/ to have opener or closer realizations. Based on literature I remember reading, I think that there is some disagreement about these kinds of conditions, though. Wikipedia says "There is no agreement among scholars on how many vowel allophones Spanish has".
I think the amount of variability that is found in Spanish is actually pretty common for a language with an /a e i o u/ vowel system.
I have the impression that the IPA phonetic values of [ɪ ʊ] are not very definitely defined. The linked Wikipedia article uses transcriptions [i̞, u̞] instead. IPA isn't really an effective tool for communicating detailed information about vowel qualities. Something like formant frequencies plotted on a vowel chart would probably be a better way of presenting information about this kind of thing.
Am I too late to answer this question?
I'm a native Hebrew speaker interested in phonology.
Most Hebrew speakers will not hear the difference between /e/ and /ɛ/.
The rest will be recognized as a "foreign accent". /ʊ/ like in GenAm doesn't exist in Hebrew and sounds odd to the native Hebrew speaker, /ɪ/ is another "monster" that might even sound like /e/ to Hebrew speakers.
Interestingly, pronouncing /i/ as /ɪ/ is a very common feature of American accent in Hebrew. for example, while a speaker of Modern Hebrew reads this line:
איש לא התעניין בזה
iʃ lo̞ itan'je̞n be̞'ze̞
Americans tend to pronounce those Hebrew vowels as
ɪʃ lɔ/loʊ ɪtan'jɛn bə'zɛ
So no, at least for Hebrew, you can't replace vowels by their laxed-equivilaent. Modern Hebrew, having only about 8 million speakers, is extremely sensitive for pronunciation inaccuracies.
The trick here is the distinction between phonemes and realizations.
To my understanding, both Modern Hebrew and Spanish have only five vowel phonemes, conventionally transcribed
/i e a o u/. But the space of possible vowels is continuous: if you throw a dart at the IPA vowel chart while blindfolded, wherever it hits will be a possible sound people can make.
So when we say they have five vowel phonemes, what we really mean is that Spanish and Hebrew divide the vowel space into five parts. Anything in the top left corner (high front) falls under
/i/, for example, whether that's
[ɪ] or anything else in that area.
In English, on the other hand, we have a whole lot more vowel phonemes: at least ten, maybe more, depending on analysis. So for English-speakers, the vowel space is divided into more parts, and what a Spanish-speaker calls
/i/ we might instead call
/ɪ/ (because Spanish's
/i/ territory is divided up smaller in English).
Finally, as sumelic mentions, IPA isn't great for representing the phonetics of vowels. It's fine for phonemes, and has lots of good symbols for that, but it kind of breaks down when you want to say anything precise about the continuum of actual vowel realizations. Look into "formant frequencies" and "categorical perception" for more info.