carbon bars are fine, as long as you check them regularly, just like with any other componentry.
i used to run an itm magnesium stem, but the faceplate bolts kept backing off. finally i gave up on it after the bolts backed off on a descent, almost causing me to crash with loosened handlebars
since then, i've been running the syntace f99 stem, which is excellent. much better design, marginally lighter than the mag stem, much cheaper, and it's made from aluminum. i run this in conjunction with my schmolke bars, which are the best, but unfortunately, some of the more expensive bars out there. if you can afford them, get them.
there is also an alloy stem sold by extralite, known as their road ul i believe. i do not recommend this stem. similar in weight to the mag stem and the syntace f99, i've seen two crack at the face place, and at the steerer tube. these things have been milled down to the point of stupidity in my opinion. extreme weight savings in this area of the bike are simply not worth the risk!