Mapping ㄱ/ㅋ/ㄲ to g/k is imperfect, and the single-double consonants are especially hard for non-Koreans to even hear the difference between as we simply don't make any distinction, but Koreans do.

Furthermore, the pronunciation on many consonants differs based on letter position in the word, medial pronunciation is markedly different that initial or final.

e.g. ㄹ might be pronounced d/l/r based on position.