Ask HN: Why Is Apple's Spellcheck So Terrible?

Especially on Mac OS?

Because from what I've seen, it seems to struggle to understand even the most basic things, flagging contractions like 'I'll' as misspellings of 'ill', making grammar suggestions that ignore tense and struggling to understand the difference between a given name and brand/series one.

These aren't issues on Windows, or in web browsers. So why are they so buggy here? Is there a technical reason this is the case?