You are saying a lot but nothing specific at all. If you really want to prove your point why don't you lay out an analysis of both pieces and prove that the technical difficulties of Islamey is more difficult than all three movements of the Gaspard. You also have the stance that even though Ravel composed Scarbo with the intention of it being more difficult than Islamey he failed. Why would you question Ravel's ability as a composer and pianist?
Ondine is not easy at all and has acrobatics which are quite difficult.
The proof is there that Ravel actually intended it to be more difficult. He could compare the difficulty of the two and create something that was more. He had extra information to create a work that was more difficult. So you are saying Ravel failed, I laugh at you for that comment.
A lot of writing but nothing which proves your point.
Fully define what you mean by Musically and Pianistic. If something is more pianistic if fits under the hands much easier. Scarbo is more difficult musically not to mention the other two movements also challenge your expression a great deal, it is also more technically difficult since Ravel intended it to be so and did well.
What about the fact that Gaspard's run time is like 2.5 times more than the Islamey? Lets take exact bars of music, what do you think is the most difficult part of Islamey and compare that to the most difficult part you think is in Scarbo. Let's see how well you know both pieces.
There are many made up personas that come up on pianostreet, if you feel insulted then prove me wrong. Instead of making up stories why don't you shows us what is the most difficult parts of Scarbo and Islamey, please contast these two sections and show why you think Islamey is so much more challenging. Exact bars of music please. I don't have to prove anything at all because Ravel's intention was there to make it more difficult and he was a great musician and certainly could realize his goals, if you understood how carefully he composed you would not doubt he really would have worked very hard to achieve what he wanted. So the burden of proof lies on your shoulders since you are saying Ravel failed at his attempt and didn't understand the piano enough to make something more difficult.
1. Composers compose pieces to be played. How difficult a piece is depends solely on the player. I never questioned Ravel's ability as a composer. No one should judge a piece just because the composer wanted it to be more difficult, it all depends on the players technical weaknesses. How can you just say he succeeded because HE said he would? That's like baking a cake wanting it to be more flavored than your neighbor's cake, and you tell everyone your intention. If you succeeded in making it more favorable, that's not by YOU saying that - it's determined by the person who EATS it. That's the same thing as pieces. Ravel didn't fail, some people agree with him. But just like the cake example, others might still find the neighbor's cake much more tasty. Is it that difficult to understand?
2. If you really think Ondine and Le Gibet are more difficult than Islamey... lol.
3. The Gaspard is a 'flowing' piece. It might be long in time, but it isn't tiring for your muscles because you move around instead of holding your hand in one strangled position.
4. You must be a terrible musician if you think I can prove a piece difficult by some few bars.

I can, however, tell you the PAGES
I found difficult. In Scarbo, the Eflat section before the Un peu retenu was one, and I found Tonjours en accelerant with the last two pages pretty hard. I say those pages mainly because a few years ago when I learned this, a minor weakness of mine was fast small to tiny cords and thirds. I also took a pretty long time figuring out the fingering of Scarbo, and I also took a quite long time to get the whole thing memorized. You can easily get the repeated singular notes done well as long as you get used to it. Same with Ondine. The main difficulty of Scarbo is to play it clear, but with a devilish feel to it. In other words, Scarbo needs much more tone color and nuance than Islamey, and that's what I mean by musically. Islamey is at its peak of difficulty after the Andantino espressivo all the way to the end. It gets harder and harder because your fingers are strangled after the repetition. To be honest, I find the page right after the slow section most difficult because of the voicing of the fast small chords. Allegro vivo is one of MY strengths. The presto section to the end is the most difficult part of the piece. This is because your hands are in one large position while moving fast, so when you reach that part, you're pretty much dead tired. And that's not it. Balakirev tells you presto, so you gotta move even faster. That's what I mean by pianistic.
Again, that's just my opinion. Based on my technique. Nothing to do with Ravel, or other people. It's the PLAYER'S strengths and weakness that determine which is more difficult, so Ravel's opinion is not in the picture.
~leethoven