I have confirmed that morphing is indeed harder than demorphing, thus solving the great mystery of Elfangor's failure to morphing away his burns in "The Invasion", and Alloran's failure to morph away the snake poison in "The Alien".
The Visitor page 12 Rachel POV: Morphing takes concentration. When you're going from human to animal, it's harder. You really have to focus. But going back to human is easier.
Evidently, Elfangor's exhaustion and burns made it too hard for him to focus well enough to morph. Ditto Alloran's rattlesnake poisoning. But going back to one's natural form is easier, and can be done even when one is injured nearly to the point of death! (As often happened to the Animorphs).
I wonder if that T.O. who lost his tail in "The Decision" could have recovered it via morphing?