.....besides the heat, the discomfort, the false sense of security.................
..........and l do not wanna look a dork!
.....and, yes, the restriction of vision.That is the major drawback.

none, it protects your head. just get a comfortable one cuz alot of them can be painful if they do not fit right.

helmet hair?

Can l suggest that you actually try wearing one? You will discover that the disadvantages you perceive are simply non-existent.

Although it is probably too late as you have obviously collided with one tree too many.