You can't say for sure if that carbon is structurally sound unless you X-Ray it.
The problem with carbon is not that it fails suddenly without warning, it doesn't. The problem is that, lots of times, you can't see the damage without specialized means, and carbon can fail in a multitude of ways that can't be seen wothout specialized equipment: delamination can be internal, bad epoxy, bubbles under the surface... most frames don't have significant defects from the factory, but can develop them after impacts.
And impacts happen everyday on a mountain bike without even having to fall: your tire can throw rocks at various parts of the frame, for example.
That doesn't usually happen with aluminium. You can easily see if it's dented or cracking if you bother inspect the frame. Moreover, dents don't make the frame fail suddenly if they're small. Most of the time they'll be fine, and when they're not, they'll probably develop a crack that grows slowly.
Most sudden failures on aluminium frame happen at welds because of welding defects, but that's quite unusual, and most of the times you can see the crack grow before failure anyway.