Getting an Andalite tail isn't really that grave IMO. And also, as far as i know, he had the other Andalite (forgot the name) treat him here on Earth. While Elfangor got his ship shot by thousands of Dracons of the Blade Ship, THEN crash land on Earth... not to mention while burning in his already damaged to the core mini Fighter ship. THAT is fatal.
And, not really, as in some books pointed out, morphing takes powerful concentration AND the energy like you've run in a sprint. Maybe Elfangor was running out of energy needed to morph.... like when we are in loss of blood, etc, how humans get weak in those situations.