Good rule of thumb is "If the developer thinks it's hard? It's too hard. If the developer thinks it's easy? It's probably just right".
I did the platforming sections in our game using that philosophy, making things honestly probably too easy? (I leaned much more on the side of easy). But then there are sections like the downward mushroom that people are still struggling with, despite that being something I thought was just challenging enough y'kno!