Those are traditionally dangerous since they tend to compile poorly; not as bad as autovectorization but not as good as just writing in assembly. And since vectorization is platform-dependent anyway (because it's so different across platforms), assembly really isn't nearly as bad as it sounds.

Though it's certainly gotten better, the reason people push those is that they're written by compiler authors, who don't want to hear that their compiler doesn't work.

Some of the reason for this is that C doesn't let you specify memory aliasing as precisely as you want to. Fortran is better about this.