Well, the good news:
On one hand, I can identify and present very clearly exactly why this cadenza is absolutely remarkable in its severe demands. On the other, I can proudly say despite some serious work it refuses to back down. Usually when you prepare music like this for an occasion you make sure you do what's necessary so that each time you look at it, it's like nodding your head to a domesticated iguana or something. But this still feels like an untamed dragon that is bigger than me.
But when i started, I wondered, well i'm certain it's difficult, but why?
For me, there's nothing technically demanding in this at all. Big claim, and then, so what's the problem is the next question. I'm addressing the colossale to the entry of the orchestra as the section in question, to be clear.
Technically demanding and how that concept relates to note perfect execution is difficult to clear up. I don't feel a single part of this is beyond the current abilities, but the first 4 bars of the colossale are extremely difficult to play clean. If there's a smudge or missed note, 90% chance it's in the g minor arpeggios. Then the other area that draws attention is the e minor area, specifically 4 bars also. This one is very very difficult to play perfectly clean. The mistakes originate from a incessant leaps in both hands and very problematic crossings. The mistakes that occur are typically always in unexpected places. In other words, always a new place. That, to me, says a lot. From my point of view, the mind must be completely all over the motions, notes and execution. But the passages are very demanding, and the result of that is massive demands mentally. The inevitable consequence is mentally being pushed to the limits and any small falter leading to issues. Or, if you become tired, then the mistakes surface. All he focus can go into a length of several bars, and then you need to breath.
Other things to consider, acknowledging the emotional nature of the work and the material leading up to this part, it's also a factor. Judging the approach and managing a tempo. A bad choice of tempo changes things very very fast. Go and look at the YT video of Yefim Bronfman and what tempo he takes the colossale. Bloody unbelievable, what on earth was he thinking. Once the 4 bars of the initial g minor area pass on, looking at the score if you look closely enough the hand crossings are not there anymore, distributions and textures are modified. No longer the same challenge.
As for the rest, the 2 bars approaching the g sharp minor area use to be an issue, time settled them for me, not a concern. The g sharp minor area used to be an issue, still sometimes traction is lost on the scales and power lags, but typically it's pianistic enough to allow for for some choice blood and thunder. The rest after is brilliantly written and pianistic with chances to rest, execution is fine.
Got to work on a Kawaii grand the other night, it had a beautiful deep heavy sound, a real pleasure to play.
I've probably put 30 minutes to 40 daily into this section, most of it is trial work not problem solving anymore although i did reconsider some fundamental approaches to the first 4 g minor bars. I don't work on the rest of the movement at all currently, and need to spend some time in a coffee shop studying for a few hours, that's all that it'll need. My time otherwise is occupied with the 2nd Hungarian Rhapsody, lots of music to learn but it's mostly fine. Liszt was a genius.