Having played around with rucksack angles, largley because the straps kept falling off and i had to re attach them, i can tell you this.

Flat straps can run straight up the back and roll over the angular neck muscles on the shoulders digging in on the inside of the strap. If they are spaced adequatley and the chest strap is tight they can be quite comfortable and flat.

Angled straps can work to stop the inside side of the strap carrying the load and thus digging in. These straps are usually very curved. Because the strap run up from the pack, over the shoulder and back down the angle can be more of a hinderance, the weight and angle making the outside of the strap dig in, especially if the straps are attached close together.

Narrow distance between attachment points of the straps can mean that the straps ride onto the neck muscles on your shoulders.

Wide distance between the straps can mean that the staps are too far overand pull the arms back.

The best set up i ever had was the strapssewn to the hip belt (frameless bag) so they came directly down the torso, eliminating the pull on the arms. Cant remember the angles on the straps.