Comment by Galactic Navy Officer on B P (Ult1m4)

@B P (Ult1m4) It's not just about having a restoration/healing magic -it's also about the magic's level and the user's strength. If the injury or illness is worse than what the power can handle, then the power can be overwhelmed. If the user is badly weakened, they can't use their power effectively. A weak or low-level user can be overwhelmed surprisingly quickly. There's also having the experience to know how to deal correctly with the problem, or even recognize it in time in the first place.

(There's an old D&D joke about who heals the healer.)

Over in the ElfQuest comics (most issues readable on their site), in the Shards war arc there's two healers. (Besides Winnowill, but she's the enemy and a bi***.) 

Leetah is much, much stronger than Mender, even though he was literally named for inheriting his healer grandfather's power. She also more experienced, being around 600 years old when he was born. When a human ally's hand is injured saving a member of the war party, Mender can only keep her from bleeding out or suffering permanent tendon damage; several days later, Leetah (who'd been off with the half of the tribe finding a safe place to live after the war) heals it fully in just a few minutes. 

(Why the weaker healer went with the war party is a long and spoiler-filled story.)