I was about to book my flight in Mexico too this morning when I found out about this! Good luck! I really think you wont have issues as it seems most people have no issues, I am just a bit of a chicken     I spoke with Air serbia and they told me the flight they have on thir website for the month of June are 100% confirmed, but any flight after that, even if it seems available for booking is not confirmed, but they will be updating their website daily.   Also I just got a res