The only perceived disadvantage of taking 10 units in year 12 is that there is no kind of backup (for example, one or two additional units) in the event that a student's performance is unfavourable in a particular subject, meaning that, in such a case, all subjects will count towards a student's ATAR, irrespective of their performance in each subject. This means that, if a student were taking more than 10 units, the effect that unfavourable performance in a particular subject would have on a student's ATAR can either be halved (if the student were taking 11 units) or completely negated (if the student were taking 12 units or more).
Of course, such an outcome is avoidable, provided you are determined to perform at a high level across your subjects and can demonstrate this throughout year 12, including being diligent in your studies, which involves regular preparation and revision in your respective subjects. This notion is supported by the high number of past HSC students who, despite the fact that they took only 10 units in year 12, were able to achieve great results, and ultimately, their desired ATAR.
I hope this helps!