You're right to point out that English pronunciation varies widely across regions, but that doesn't fully negate the value of a systematic orthography. What germandiago is referring to is the relationship between graphemes (letters) and phonemes (sounds). Spanish has a highly phonemic orthography, meaning the rules for converting letters to sounds (and vice versa) are consistent and predictable. Yes, there are accentual and dialectal variations within Spanish (e.g. seseo in Latin America vs. ceceo in parts of Andalusia) but these are largely phonological shifts applied systematically, not random deviations from spelling norms.
In contrast, English has a deep orthography, where historical layers (e.g. Norman French, Old Norse, Latin borrowings) and sound changes (like the Great Vowel Shift) have led to a chaotic mapping between spelling and pronunciation. A consistent system wouldn't eliminate dialectal variation, but it could reduce ambiguity and aid literacy, as evidenced by languages like Finnish or Korean.
I don't know if Korean is ultimately that good. Hangeul are a monstrous improvement over the old mixed script (which itself is better than the Japanese iteration because the Koreans only used Chinese characters for Chinese loans), but it still has a lot of sound change rules and can be a bit of a pain to read because of how letters flow to the next syllable. It's not in the same league with Finnish or Spanish, at any rate.
In contrast, English has a deep orthography, where historical layers (e.g. Norman French, Old Norse, Latin borrowings) and sound changes (like the Great Vowel Shift) have led to a chaotic mapping between spelling and pronunciation. A consistent system wouldn't eliminate dialectal variation, but it could reduce ambiguity and aid literacy, as evidenced by languages like Finnish or Korean.