If your upper body fits within the 'trampoline' mesh back panel so that there is no rubbing or pressure points from the edges of the back panel, it is a comfortable backpack. I have not really found it to be superior in comfort for back issues over all other backpacks, but it is comfortable.
Outside of someone not 'fitting' the mesh back panel, the largest negative is weight, as you pointed out. It is double the weight of other quality backpacks around the same price point, which offer similar performance, comfort, and sometimes with better usability. Of course, if you are transporting the bag, weight is not an issue
The capacity (not size) is not really an issue, other than the added weight due to more material used.
Some unique things about the Atmos, and some other Osprey backpacks, are the methods they developed for adjusting the torso length with the shoulder harness assembly, and the hipbelt sizing.
Their torso length adjustability makes fine tuning the torso length fairly easy for those who are 'in-between' torso sizes of Osprey backpacks. The hipbelt adjustment tends to eliminate the need to purchase and install a new hipbelt if you go from a size Small hipbelt to a size Large. And vice versa.
There is nothing wrong with the Atmos, and a lot that is right. That same assessment would apply to a number of backpacks, too.
Feel free to contact me if i can be of help.