Only a near perfect execution can carry a Metroidvania game over 30 hours long (triple the length of the founding classics), and Team Cherry pulled off the impossible. The genre lives and dies on three key elements: exploration, combat and platforming (in that order IMO), and Hollow Knight excels in all of them. Aside from DLC super bosses, the game keeps a good balance between challenging and fairness, so you should not be deterred by its vastly overstated difficulty. Special shout out to the world building, which deploys the same esoteric method of Souls series, while creating a vivid sense of space and history.