This is a question I've struggled with recently.
My game https://vigilantgame.itch.io/vigilant has one hard difficulty level. This is because I thoroughly enjoyed the Dark Souls games, and had a great sense of satisfaction when I finally beat it's ridiculously challenging boss fights.
When it comes to having only one hard difficulty settings across the game, I can think of the following pros and cons:
|-Beating it gives a great sense of personal accomplishment|
-Gamer street cred for winning
-Not feeling like a n00b for having the option to make it easier
-You'll only get the cream of the crop getting to see your awesome ending
|-Might be too frustrating for players to finish, so many will quit|
-Can't adjust the experience on the fly
-Limited re-play value as can't do a 'newgame+',since you already beat the one difficulty