In Latin, it is thought (as far as I know) that within a single word, /ns/ and /nf/ were always preceded by a long vowel.
This is a somewhat complicated result of a hypothesized sound change in words containing /ns/ or /nf/. The original sound change is supposed to have turned Vns and Vnf sequences to [Vːs] and [Vːf] sequences (or possibly [Ṽːs] and [Ṽːf] sequences; the exact realization and occurrence of nasality as a contrastive element for Latin vowels is somewhat debatable). According to Allen 1978, consonantal [n] was later re-inserted before the fricative in the pronunciation of Classical Latin while the vowel remained long, resulting in [Vːns] sequences (Vox Latina 2nd edition, p. 29-30). In Classical Latin, long vowels occurred even in prefixed words that started with in- or con- followed by s or f.
In Greek, the cluster /ns/ apparently was similarly unstable, but it doesn't seem to have become strictly eliminated by sound changes, and as far as I can tell Ancient Attic Greek differs from Latin in having a fair number of words where this cluster comes after a short vowel. My basis for the preceding statement is the spelling in words with epsilon-nu-sigma; I'm asking this question to learn whether the spelling is misleading, and whether we know the length of alpha, iota and upsilon in this context.
For example, the prefix ἐν- seems to occur fairly freely before σ or ζ. As far as I can tell, this prefix doesn't have an alternative spelling like εἰ- in this context, even though it does have alternative spellings ἐγ, ἐμ to represent place assimilation to following velar or bilabial consonants and ἐλ, ἐρ to represent full assimilation to a following liquid consonant. I would think that this indicates that Attic Greek words starting with ἐν-σ- or ἐν-ζ- were in fact pronounced with a short vowel [e] followed by a nasal consonant [n].
But I'm not certain, because Ancient Greek orthography seems to have indicated assimilations or sound changes only sometimes, not always. For one word spelled with ΝΣ—the proper noun Τίρυνς—Wiktionary suggests that the accentuation implies that "the standard Classical Attic pronunciation" was actually "Τίρῡς".
What is the general distribution of ΝΣ in Attic Greek, and what do we know about its pronunciation and the pronunciation of preceding vowels?