I did Unit 1/2 Biology in Year 10, and Unit 3/4 in Year 11, so I might be able to give my perspective on it.
There are a couple of advantages to it. For example, I had a headstart to finding out which field of Science suited me best. I enjoyed the subject very much and gave me some idea of what I wanted to do at Uni - Health Sciences. Being able to do Unit 3/4 Biology in Year 11 also gave me an opportunity to choose other subjects in Year 12 (Now that Biology is finished, I can choose Physics). Having done a wide variety of subjects, I have a better understanding of Science as a whole and has definitely given me an advantage at Uni.
Looking back at Year 11, I saw that there were some disadvantages of doing Units 3/4 in Year 11. This may not apply to everyone, but has happened to me. Working in a Year 11 environment, I didn't see the importance of a Year 12 subject - took SACs as normal Year 11 assignments, not knowing that VCAA exams reflected the type of questions in the real exam and what grades/study scores to aim for. I revised for the exam like how I would do for my other Year 11 exams. As a result, I didn't do as well as I have expected if I did the subject in Year 12. With a bit more advice and support from teachers, the problem could have been overcome.