[sago]

There are more than 1500 NT hapax, and epiousios (and others) aren't exactly hapax (they appear twice ['dis legomena'], or more, though often in bits arguably copied from one source). A minority are names (e.g. Epicurean only in Acts 17:18), most are words that appear in non-NT texts (technically hapax, but often meh, obviously), most others (like 'epiousios') are compound word that are easy to understand (e.g. if you read an English writer saying 'she metathought about the problem' it wouldn't mystify you), but that still leaves plenty of oddities.

But the juicy ones have theological significance. If more significant, even simply compound words can generate debate, even words known outside the NT ('What did they mean in this context?'). Off the top of my head: Theopneustos (a hapax) from 2 Tim 3:16 is the forceful example (forceful for evangelicals at least). Authenteo in 1 Tim 2:12 is important in feminist theology. Arsenokoitai in 1 Cor 6:9 is a crucial question in LGBT theology (a dis legomenon, strictly, [oh hai 1 Tim, again], though I guess this post is allowing those ;)

Tanakh hapax are more difficult because we don't have masses of other/earlier Hebrew and Hebrew is less often based on compound morphemes. So 'OT' hapax are more widely known, imho.