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Alu and a lot of its Alloys are crystalline i.e. brittle. Hence the thought that frames are 'harsh' or 'stiff', no?

No.

1. All metals and their alloys are crystalline*
   
2. This doesn't inherently make them brittle
   
3. The brittleness (or otherwise) of an aluminium alloy doesn't affect its stiffness, for all practical purposes all aluminium alloys used for cycle frames have the same stiffness.
   
   

Aluminium frames made from modern high strength alloys exploit the strength to allow large diameter tubes, which make the frame stiffer at a given mass e.g. doubling the diameter while halving the wall thickness makes a tube about 5 times as stiff without changing the mass per unit length. You can only make tubes with that kind of extreme ratio of diameter to wall thickness from strong materials, otherwise they buckle.

*Metallic glass hasn't made it to bicycle parts yet, so we'll ignore it for now.