What are pop-rivet aluminium security bars vulnerable to?
what attacks are aluminium security bars vulnerable to that steel bars aren't?



Aluminium is a lot softer than steel and is quicker to cut through using a hacksaw.
There are hardened grades of steel that considerably increase its resistance to cutting compared to "plain" steel. 
The same diameter bar in aluminium is much less strong than steel and so more vulnerable to bending, stretching and shearing.
Aluminium is a fairly reactive metal but your security bars are probably not going to be attacked chemically by a would-be intruder.