I think once you have the need to go deep enough into a topic they all get hard. Debugging and testing is also hard in all things that are somehow related to r

I think once you have the need to go deep enough into a topic they all get hard. Debugging and testing is also hard in all things that are somehow related to realtime or concurrency. E.g. OS development, embedded firmware, network stacks, etc. For these things you often also need to know about Math, Physics, Statistics, Electronics, Hardware and Software Architecture, etc. Game engine development is also hard because you should also know about most of this stuff to really find the most efficient solutions.