What is the hardest kind of software to build and maintain?
I'm hoping to get some objective insight and arguments, not personal preferences.
I think it depends on the temperament and interest of the developer. Different people find different sorts of work easy and hard, and there is an enormous variety of types of software work out there.
The kind that is meant to automate manual processes that have evolved over decades by adding edge cases, to accommodate single business partners, that are triggered by nondeterministic heuristics that are undocumented. In other words, any back-office software.
Restaurants, pub, hotels, vending demands support any day, any time.