I disagree a bit - I think for a piece that is as "heart on sleeve" as the Elgar, she was temperamentally well suited to it (the Dvorak Cello Concerto too), but for other works she was less satisfying, such as Beethoven or Brahms. Sure, it's chamber music she's doing in those composers, but still a valid criticism.
In fact, Rostropivich once said something similar when an interviewer asked him why he never recorded the Elgar. Something along the lines that he could never play it as well as du Pre, given their different temperaments and life experiences.