Playing a Mellotron is like playing no other keyboard instrument on the planet.

It's not keyboard technique that counts, it's Mellotron technique. You have to pay attention to each note for the entire time that the key is depressed and even a tiny lapse of concentration often means a wobbled note or a tape hard stop. It's all about listening to what you are playing and thinking about it as a group of thirty-five individual instruments rather than as an instrument in and of itself.