iserith
Magic Wordsmith
I might argue that it isn't, actually. At least not from a player's perspective. The character probably seeks that, in-character, but the player seeks complications and difficulties that they can then overcome (or fail to overcome) to show off the personality traits and fantastic abilities of their character.
Climbing the ravine on the way up Mt. Pudding is the complication and difficulty to overcome in this example.
The player wants to roll dice and do math to defeat challenges. An easy climb to the top, while it might be what the character has in mind, probably isn't satisfying for a player (too easy, too uninteresting).
Let's not confuse the example again as I did upthread. The ravine is likely one of a number of challenges on the way to the peak of Mt. Pudding. I would add that rolling dice isn't typically to the player's benefit in a game where the GM decides on success, failure, or uncertainty. (The latter case is when we roll dice or otherwise resolve with some mechanic.) Hoping to get lucky with the dice isn't a good plan. Striving for outright success is better.
The old example of failing a social interaction so that it degenerates into combat is handy here - when the incentive is toward combat (either directly, through things like XP, or indirectly, through robust systems that allow the player to control dynamic interactions), degenerating into combat isn't a punishment, it's more fun for the player (even though the character might've sought to avoid that).
Difficulty itself isn't a disincentive, it's something you WANT as a player.
At least until it becomes so difficult that your goal can't be realized.
But the examples of Fail Forward so far don't seem to comfortably accommodate a goal that can't be realized, so that level of difficulty would also seem to be off the table, if you're applying the design logic of Fail Forward.
I think some more issues are getting muddled here. Of course difficulty is desirable - it's part of what makes a challenge satisfying. And being desirous of interesting success and failure conditions is good and I share that (again, that's just stake-setting which is all fail-forward is!), but as some have stated, they don't care if sometimes things turn out not to be so fun as long as the net fun over the long haul is positive.