I don't think adaptions from a manga (especially if compared to video games and novels) are actually that difficult when the animation studio has the time and money to do a good job. Some adaptations are nearly identical to the manga (an example: Monster). But with videogames it's different, since a completely faithful adaptation is almost impossible due to the nature of the media itself. Imagine what a nightmare it would be to adapt Dark Souls and Bloodborne, and Nier Automata isn't much easier.