The hardest pill to swallow is marginal difficulty. Everyone can agree that writing something like a modern full fledged browser is hard. But how hard is it to add a small feature to said browser? Well, the feature in isolation is easy, but the entire browser at that scale is much more complicated than the sum of its parts, so the difficulty comes from incrementing the complexity of the entire browser and not from how hard the feature itself was. Good code design can alleviate this a bit, but not entirely.