Sorry, my bad, I did not read this article. Zach himself is calling the game "stupidly difficult". I still strongly disagree with the term though :) Plus seeing the graph, I find that it does not seem to be the case - I mean most levels in the main campaign seem to be over 80% success rate (hypothesis: "defense" levels are not available in the Android version, which could be a reason why they seem to have a lower success rate)

80% completion rate with 20 levels works out as a ~1% chance of ever finishing the game (which i'd say counts as fairly hard). Looking at the graph, the pass rate seems to be higher, but there are also a lot more than 20 (non-optional) levels, so I would guesstimate the finish rate to be less than 1% (90% per level for 40 levels is also around 1% to finish the game, for example).

https://steamcommunity.com/stats/SpaceChem/achievements according to steam achievement "beat the game", it's actually 4%. I do agree that it makes it a difficult game to finish. Then again, my point is that I think it's fine not to be able to finish a game, especially a puzzle game which have no story. You just have to spend more time on the puzzle you are stuck on, so in a sense, you get more game time for your money :)

SpaceChem does have a story though. I gave up on the game after it got too difficult, and at that time I was disappointed that I wouldn't find out what happens in the rest of the story.