This is one of those things that newbie game programmers seem to underestimate, frequently.
And when I say “newbie” I include “experienced, skilled programmers with 10+ years of coding experience – but who haven’t yet shipped a game of their *own*”.
(e.g. I’ve seen a couple of studios that started as Digital Agencies, or as Animation Studios, etc – that then transitioned to writing their own games. This is the kind of thing that they often struggle with. Not for lack of skill or general programming experience, but for lack of the domain-specific experience of game coding)

I'm primarily a server programmer, but I still end up doing UI work every once in a while, and that's been true for the majority of the last three weeks. Not being an experienced old hand at UI design, I find myself falling back on first principles and decade-old books. I'm mostly happy with the results, but the code that results in implementing the interface... well, it's not the kind of code that I like to write.