tax is one part, but paperwork and penalties are the other scary part. I have only heard of it, how minute mistakes can end up with 50K fines etc. Probably some one who has gone through the filing taxes from overseas can give better insight.
US citizen who worked in New Caledonia for 2+ years. The scariest part is that I don't have any official documentation for what I earned. I had access to our payroll system that records the amount I earned in the local payroll system -- I didn't generally have a need to convert that to USD every pay period and record it. I was paid "in kind" for housing. And at the end of it all I got an unofficial letter stating what that amount was.
It's true. There are two specific forms which have 10,000 USD late fees which can compound at least three times per year each. So doing your taxes late could cost dozens of thousands of dollars. These are specific to controlling CFCs, not applying for the FEIE though.