Carbon fiber is very safe for bicycle frame construction. The aluminum is more likely to fail than the carbon. In the past the joints failed because of corrosion, but this problem has been solved-so they say. Carbon fibers are woven into fabric and molded and set in a epoxy or resin to harden the fibers into a solid form.Is is very important not to over-tighten carbon components like seat posts or bars, or fork crowns as they will crack and then fail.
The real reason that carbon was first used was for making very lightweight bicycle frames. Now it is used for softening the ride of harsh riding aluminum, style, and coolness. Soon everything will be carbon.....pure joy