This is always said about ally frames to me it is tosh. I have a aluminium orange MTB and it far more comfortable than my steel MTB's even when I had rigid forks on it.
I own a 1980/1 Alan that is comfortable. I also ride a Pinerello Monvisio in aluminium the ride on that is sublime. You can make a steel bike ride harsh or an ally bike ride harsh. There is more to ride comfort than frame material, alot more and it is mostly tyres ans tyre pressures.
I also ride steel bikes and a aluminium/carbon bike. All of them are comfortable and ride really well I have built them that way with a complementary slection of components to the bikes intended use.
Steel frame do break, so do Ti, so do alumium and so do carbon. One material is no more prone than another.