one example i come back to is multithreaded layouting/rendering browser engines. it was mentioned in the servo blog post back then when mozilla worked on it: "we tried it in c++ but it was simply too hard and complicated" (the argument was that they were able to do it in rust). still, as long as rust wasn't avaiable, the problem was considered "too hard" for the team that wrote firefox. in my opinion that's a solid argument for "programming problem that was too hard".

but as simon said, i too consider coming up with "the algorithms, the strategy" as part of programming. saying "it's easy to do if you know exactly how to do it" is somewhat tautological.