I had to build a hotel booking platform on Amadeus a few years ago and it was a nightmare to work with their API. It would frequently break or start returning differently formatted responses. Also the support was so bad, I felt like I was the first developer the reps had ever spoken with. I hope a newcomer can unseat the incumbents in this space, it's badly needed!

You need a few legal requirements before using Amadeus (you can see them just before registering for their developer portal). And the process for going live with them - even after meeting their requirements - is so slow.

Duffel breaks a lot of barriers to entry.