I was looking at this: https://flightfox.com/contest/57321
And I checked the dates on Momondo, and it cost around $1400. (I only checked one set of dates though)
The searcher expressed that they'd pay more for a cheaper flight. Still, the cheapest the experts found was cheaper and faster than what I found on Momondo. As a matter of fact, they found it for $636!
This surprised me quite a bit, and it made me wonder: What factors make these flights so much cheaper than "standard" flights? and are there any flights it wouldn't be worth it to use FlightFox to find, where Momondo etc. would be equally good?