This is such a complex topic because unlike football, I really don't think hockey players are taking blows to the head on every play. Not to say that these guys never get hit in the head or never get knocked out, but I think it's roll specific (Take a guy like Mike Richards vs Anze Kopitar over a career). I think the goon roll (which is largely being phased out) is another story entirely. I was reading about Steve Montador, who just died a few years ago and his story is tragic in how many head injuries he sustained. Major depression due to a ton of concussions throughout his career, but I think his is specific to his roll as a fighter. The story is sad because they groomed him from the time he was about 14 years old and just kept promoting him and promoting him in that roll (I'm having trouble finding that article right now, but you get the gist of the story below).
The last years of his life are brutal to read about.
“He wasn’t my brother. He was like a different person inside his body and it breaks my heart. It breaks my heart today. He wanted to be the same guy, but he just couldn’t."
“With respect to his brain, I don’t know how many cylinders were working in there. And as the days have gone on, I think my brother knew he was going to pass away early. He never said that to me, but I just feel he kind of knew that. And as my dad said, he packed 70 years into 35.”