User blog comment:Maki Kuronami/Lesson 40: "IT WAS ME CINDER! IT WAS ME ALL ALONG!"/@comment-5217560-20160215042602/@comment-10390252-20160215140729

The big difference is that Aang knew that he was The Avatar. Although, in the early seasons, his control over the Avatar State was borderline non-existent, he knew it could happen.

Ruby, in contrast, had no idea about the possible consequencs of her eye colour. No-one had bothered to explain to her about her unique heritage or offer the slightest hint of special training in how to harness that power. So, it all came boiling out all at once and the power drain sent her into a coma.

Now, it's possible that the power can only be harnessed when it is unlocked by dire circumstance. However, I still think that it was another of Ozpin's damned secrets coming back to bite our heroes on the ass. The man needed to trust more, communicate more and have his pieces better trained. As it is, he's like too many fictional mentor figures who has the tendency to push their chosen champion into the face of the enemy and then stand back with an indulgent smile, trusting in 'destiny' to save them.