Rich Geldreich who had worked at Valve for five years shared the most difficult work he's done: Steam for Linux. That's on top of his time at Valve he worked at Microsoft, served as an adjunct professor, was a head researcher for a company since acquired by Google, was CTO for a mobile games company, formerly a principal software engineer at Unity, now an independent consultant / software engineer, and an expert on data compression.
Besides the Linux activities at Valve, he worked on Portal 2, Dota 2, Counter-Strike: Global offensive, and more. He wrote the Source Engine 2 GPU profiling support, worked on many Direct3D to OpenGL optimizations for games, and more during his time at Valve. We've written about Geldreich's accomplishments in many past Phoronix articles.
Geldreich published a "end of year realization" on his personal blog where Rich commented, " I'm basically an artist who "paints" with code, and good debuggers are one of my brushes. Except, each "painting" involves a variable amount of mental and/or physical resources. The Steam Linux project was the hardest one I've ever done so far. Getting games on Linux and pushing the GL driver teams to go in the right directions was extremely difficult."
Hopefully for those newer to Linux game development the situation is more ideal these days with now having Vulkan, the OpenGL Linux drivers have matured a heck of a lot over the past two years, more middleware and other game components are available for Linux, game engines are more optimized these days for Linux/OpenGL, etc.