I think that the difficulty in the first 3 games is derived from the character movement, in wich the jumps, for example, need to be very accurate between platforms. The abscence of checkpoints (like in the new games) add difficult to the games, too.
In all metroidvanias, except CoTM we have a merchant, to who we can buy potions and items. I mean, if there any problems in our equipment or items, we only need to slain some enemies to gain money, so we can buy more potions, armor, weapons, etc. In CoTM we don´t have a merchant, so we can´t buy items or equip, only by defeat enemies.
In other words, I think that modern games (or gaming) we have easiest ways to finish its, being longer than the old ones. A game like Dark Souls its difficult, but we can use items, too. But, as always, its depends in the "experience" of a gamer...