Ori and the Blind Forest is difficult but not frustrating, being balanced around a mechanic where you can essentially save your game at any time outside of a few specific instances. Save scumming is not only encouraged but part of its core design. I haven't played many 2D platformers other than it since the 16-bit era, so it's difficult to for me to make comparisons to Nintendo platformers. I would say the game is as difficult as anything in Super Mario Bros 2 - The Lost Levels or the lost world stages in Donkey Kong Country 2 & 3, but because of the saving mechanic it's less likely you'll hit a wall with progression.
Though the game is structured like a Metroid or Castlevania game, its difficulty lies in its platforming rather than its combat. Its combat is really weak and is at its best when it exists only to serve the platforming - think bouncing off Koopas to get to higher platforms in Mario; this is how combat is best treated in game. I feel like it's a really unique take on the genre, since most of these styles of games focus on combat and character progression through getting new weapons and abilities, but in this most of your meaningful progression is tied to abilities that affect your mobility through the environment. The moment to moment gameplay reminds me more of Super Meat Boy than any other 2D platformer or Metroidvania I can think of.