For native English speakers, Korean is generally considered to be one of the most difficult languages to master despite the relative ease of learning Hangul. For instance, the United States' Defense Language Institute places Korean in Category IV, which also includes Japanese, Chinese (e.g. Mandarin, Cantonese & Shanghainese) and Arabic. This means that 63 weeks of instruction (as compared to just 25 weeks for Italian, French, Portuguese and Spanish) are required to bring an English-speaking student to a limited working level of proficiency in which he or she has "sufficient capability to meet routine social demands and limited job requirements" and "can deal with concrete topics in past, present, and future tense."
I assume the difficulty goes the other way as well. I guess this explains why, despite English language education being so emphasized in South Korea, Korean students generally don’t reach fluency – in middle school I knew classmates who were spending 5~10 hours every week in English classes inside and outside of school, but barely reached conversational level