There are numerous factors that affect the harshness/softness of the ride of a bike: frame material, alloy and frame design.

Some people stay away from aluminum because they believe that it produces a harsher feel. I don't have enough knowledge or experience to justify a response to that. Generally, the less expensive road bikes are made of aluminum because it is a relatively inexpensive material. I think that I am correct in saying, all things being equal, aluminum is less flexible and more brittle than some other materials but you also have to take into account the alloy of the metal as well, some are softer than others (as well as many other properties of different alloys).

Given that aluminum is common in less expensive frames and aluminum tends to be more brittle than some other frame materials, consider this. The Salsa Warbird frame is aluminum, not an inexpensive bike and the seatstays, despite being made of aluminum, are designed to flex and thereby produce a softer ride.

There are many factors that affect the ride of a bike, design, alloy and material are only three and all three can be played with to produce different types of rides.