I've always used in-ear headphones but also always felt fatigue from the pressure.
But the AirPods just don't have it. It's more like they sit at the edge of your ear instead of burrowing inside it, and they explicitly use some kind of vent to equalize pressure as well.
Visually, these look like they operate in roughly the same way.
I use IEM because the regular old plastic ones that sit in the ear give me actual physical pain after 5 minutes of them just sitting there, it's incredible. And the only brand of IEM that don't fall off are BOSE ones, which are oval shaped and have that little stability wing like these ones from Google.
In my case, the most comfortable tips came from a no-name cheap pair. So I guess it is one area where fit is more important than anything else.
http://tinyurl.com/yb6qfpes (Koss over-the-ear headphones example)