I don't have enough experience to know this for sure, or rather to back up my opinion with a bunch of examples, but the common wisdom is that all-aluminum frames can be harsh rides because of the nature of the material... aluminum is very prone to metal fatigue and flexing induces that, so designers build all-aluminum frames very stiff to prevent flex.

This might seem like a small thing, but a large part of riding longer distances is ability to tolerate discomfort, so, the less the better. A harsh, buzzy ride would seem to be fatiguing.

Enter carbon stage left. Carbon can be engineered to flex very predictably, and more or less indefinitely. Aside from occasional asplosion. So, our savvy bike designers, designing to a price point, will use carbon at certain key spots- usually fork, seat stays, seat post- to alleviate the harsh ride of the stiff AL frame.

On my bike, a Specialized Sequoia, this approach seems to work quite well, as it's a relatively smooth ride.

Anyway, that's the theory. I'm certain there are numerous examples to contradict it- smooth AL bikes, harsh bikes with carbon bits. So the smart money says- ride 'em and see.