"That said, Keith was a huge talent, and a great player, even if he didn't like Mellotrons."
Treu!! He didn't like Mellotrons (or Chamberlins) because they couldn't play quickly (at least that's what he thought).
He very much did want a Birotron though, because the keyboard itself allowed for fast runs, but despite his friendship/clout with Wakeman, he never received it.
Incidentally Todd Rundgren didn't want a Birotron because (although he'd never seen it) 'it couldn't play fast enough'.
He was basing this on the Chamberlin he had.
We know these opinions are borne of ignorance. They have more to do with the keyboard being out of adjustment on well worn and and maladjusted units than anything else.
When the keyboard is properly adjusted, they can play fine.
A listen to Elton John's Mellotron flute solo in "Lucy In The Sky with Diamonds" proves this.