The ATAR cutoff being higher means that there is higher demand. Much like the price of something. The more demand there is for it, the higher the price.
A higher demand does not necessarily mean it's 'better'.
The difficulty of the course really depends on how often you do your revision/study, but I will say that is has the potential to be quite difficult especially in higher years, even with regular study. Regarding dedicating a lot of time to it, it depends on your goals. Do you just want to pass? Do you want HD average?
USYD does indeed offer combined BSc(Adv Maths) and BArts, provided you have the right ATAR. You will first need to apply for the normal BSc/BArts and provided your ATAR is sufficient for the Adv Maths stream, you will be placed there. Link:
http://sydney.edu.au/courses/programs/science/Bachelor-of-Science-and-Bachelor-of-Arts
As for the University... UNSW teaches it in a slightly different manner to USYD.
USYD teaches at a more rigorous level and goes into a LOT of detail. It does not cover as much content, but the content it goes through is extremely thorough and detailed.
UNSW teaches at a slightly less detailed level, but they cover a LOT more content, especially in 1st Year.
So think about spreading butter on toast. USYD is spreading a large amount of butter on say two slices of toast, whereas UNSW is spreading it across say 3 slices of toast (so ofc the amount of butter is not as thick).
