Body awareness.
Music theory is often taught quite poorly, so you have a ton of musicians who feel they don't have the talent for it but maybe just had poor instruction.
Music theory involves a lot of knowledge and a ton of practical application to learn properly though, so I'd say it's reasonable to struggle with it.
Music theory is a necessary thing [I should hope everyone agrees, whether they are aspiring composers, or improvisors in the classical tradition, or merely performers who seek to understand], but it is indeed a necessary condition that to learn music theory, and to practice it, there should be a struggle.
One unnecessary thing I did for ages was worrying too much about movements I found difficult. The trouble is that it became a never ending obsession. I worked on a given movement until I could do it, then easily found another I could not do and worked on that for weeks. It went on and on for years until I could play all sorts of movements which would be used in less than one percent of my playing and had minimal musical implication. Having the Virgil Practice Clavier no doubt aggravated the neurosis. I have more sense now and work on things that I can see will likely enhance my improvisation. I've certainly never seen the point of it for my music and at seventy-five probably never shall. The lack could be mine, but I have never understood how it could help my personal musical creation at all. My teacher in my teens did his best with me but the poor man gave up in the end. I think we might have had this conversation before in another thread ?
I think it's necessary if you want to be able to imitate classical idioms, or understand already written classical pieces, but if all you want is to create in a style thats completely, intuitively your own then I do not see the point for it either.
However, in the end I still preferred my “wrong” sounds to his “right” ones so saw little point in continuing. At least I did make the effort I suppose.
Yes that’s it exactly. When I was about twelve I adored Beethoven’s music ( I don’t now but that’s beside the point), saved my pocket money and bought Tovey’s analysis of the sonatas.
If your definition of "pianists" also includes the infinitely more numerous beginning and intermediate piano students (as opposed to the far rarer ones who make it to the advanced level, let alone collegiate, let alone who win elite competitions and/or make a successful career out of performing), then without any doubt, the most unnecessary thing pianists struggle with is inefficient practicing. A failure to demonstrate metacognitive skill at the piano. I wrote a series of articles on this, with the first article starting here: https://blog.twedt.com/archives/3812