Carbon bars are stems are madness. They don't save any meaningful amount of weight over lightweight aluminum ones. The amount of money spent on them could be spent on pretty significant weight-savings things like forks or wheels.

They carry a high risk of catastrophic failure. Even if they don't fail catastrophically, then they're still extremely fragile and prone to breaking from something as simple tipping your bike over, never mind crashing.

Some people say that they dampen the effects of the road, but so does cork tape, or gel pads underneath the cork tape, gel gloves, and carbon forks.