Landing in Incheon was a bit of a nightmare. The foreign entry line I'm certain had a thousand people. It took about 2-3 hours just get past that so it ended up being close to midnight. Even though I had everything planned and got my SIM card and transportation card with no issue at the airport there were so many obstacles. Only about 2 of the ATM machines were working and maybe 4 of the bus ticket machines for the whole terminal. The ATM machines did not recognize any foreign cards and the bus ticket machines only accepted the transportation card. To add money to your transportation card balance you may only use cash and this is true for everywhere in Korea and Taiwan. So the only option to take the bus out of the airport for me at the time was to go to a foreign exchange teller and exchange dollars. Even taking the bus would have got me to the hotel at maybe 3am. I ended up getting an Uber because Korea's versions of Uber doesn't take foreign cards either.
When I got to the hotel I recognized the ATM-like machine in the lobby. They have these machines that give foreigners a pseudo Korean credit card to use and the machine accepts foreign bills, Paypal and foreign credit cards. Of course there's a fee and a bad exchange rate attached. I saw lines for this machine at the Korean version of Times Square.
On top of all this I had to go to 6 different convenience stores to find an ATM that recognized foreign cards. The only one that worked was outdoors too so there's always a risk there. I ended up withdrawing all of the currency I felt I needed for the entire trip from that machine.
So fair warning to bring at least some Korean Won with you before you travel.