Challenging fun is the kind that defers satisfaction to near the end of the process - so the more challenge there is, the more uneven the satisfaction is likely to be. It's the same satisfaction one experiences with language fluidity, and being able to "converse" with the mechanics. That is the cause of an essential problem in the design of such games: enjoying the game means becoming literate in what the game is doing. Some people are hooked on the pattern recognition particular to that form of challenge and find it easy to progress and satisfying to win. Others have difficulty maintaining attention, get frustrated quickly and quit. This is evident in reviews of UFO50, the anthology of "authentically fake retro games" from the makers of Spelunky. Most of the games in UFO50 are difficult in more-or-less the same ways that games of the NES era were, with some intentional anachronisms. People find games they love and games they hate in the collection, but their opinions on which ones, and how hard they are, are all over the map and in vigorous disagreement. It is an excellent litmus test for what kind of gamer you are.

The most notorious game in the collection by most estimates, Star Waspir, is a vertical scrolling shooter. For most people, it's the hardest thing they've ever played, but they also like it if they persist, and the overarching goal of completing all 50 games propels them into developing appreciation. The enthusiasts in vertical shmups, on the other hand, find it a bit out of touch with where the genre is and not all that hard relative to other games: the mindset of shmup players is one of playing the same 15 minute experience repeatedly with incremental improvements in progress or score over weeks and months, and intentionally choosing between easier and harder routing according to their current skill - as opposed to the mainstream of continual progression through content with a binary conclusion of "beat the game/did not beat the game". Star Waspir has elements of the modern genre but it's also stripped down to be more within the 80's vintage, retaining certain rough edges.

A large part of what hooked people with HK was that everything was "paced for mortals" and stayed in an accessible Goldilocks zone with a lot of room to grow into doing harder stuff. This also made it incredibly boring to Metroidvania enthusiasts who knew all the tropes: it's the plain vanilla version of this gameplay, given a lot of attention to detail, but it takes a while to get going and doesn't have many things for enthusiasts. Silksong has pushed a little more into the enthusiast territory, which is always going to be to popular detriment.

I don’t think you can compare UFO50 and Silksong.

UFO50 is voluntarily retro and purposefully use poor design as a kind of homage to video game progress and as a way to foster nostalgia. Part of the pleasure is seeing the changes between the games pretending to be older and the newer ones. Plus, it’s pretty clear it wasn’t made with the idea that most players would finish all the games. Part of the pleasure is sampling the large collection, closing games you don’t like until you find one that sticks. UFO50 objectively contains games which are not that good next to real gems but it is particularly brillant as a collection. Plus independently of how it plays, it’s interesting in itself as a kind of experience. That’s indie at its best.

Silksong is supposed to be a modern game and excels at some parts but the issue is that some design decisions are just, well, bad. I agree with the person you are replying to that it’s not particularly respectful of its player’s time for exemple forcing you to grind and slog repeatedly through uninteresting parts to actually play the fun parts. This is notably something FromSoft entirely solved a long time ago. To me that’s entirely orthogonal to being difficult. It’s an enjoyable game but a flawed one which makes for a tarnished experience.