We're talking about concertos here. Not etudes. Often in a concerto, the more notes the more complex, especially in Rach 3 and such concertos. I'm not saying other instruments are less difficult to play at all, I'm just saying if you play piano you have more music to think of at the same time. Of course there are things in the piano repertoire much less complex and with much less notes than the most complex cello pieces. And I'm not talking about using more fingers at the same time. It's musical. A conductor has to think about more music at the same time than an instrumentist in the orchestra. It's the same thing ...
well, if you think so, i respect your position, but i disagree completely.
btw, rach 3 is - in spite of its ridiculous technical demandings - quite straightforward musically. i'd not think of it as "complex" even in a million years. furthermore, i used the etude example just as that: an example. the same applies for concerti. some that come to my mind are schumann's or mozart's 20th. difficult to play (and i must be honest and say that i don't play well either), but very easy to memorize.
about the conductor point of view, i have worked as both vocal and chamber conductor, and i have the same to say as to instrumental music. although i agree there are more lines to consider, it's not always related to the difficulty of memorization in my opinion.
getting back to the point, learning a concerto in one day it's very very hard, in spite of what instrument you consider, don't you agree?
best!