Hey! So I've actually just completed my first year of this course. Obviously I won't be much help with respect to providing information on employment rates and such, but I just thought I'd point out a few general things about the course.
Each year Monash runs a program called ResearchFirst, where they offer around 25-30 summer research scholarships exclusively to students in the advanced research degree that have just completed their first year. A wide variety of projects are available; there were quite a few physics and astro ones this year iirc. It's a great opportunity to get some research experience nice and early imo. As part of the degree you're also required to undertake 'research' project units, which instead of sitting through lectures and reading textbooks to learn, you actually do a mini research project throughout the semester instead. Most people begin these in second year; I haven't done one myself yet, but from what I've heard they're fantastic and provide invaluable experience.
Apart from these and some other small things (having a nice small cohort, having to double major, compulsory honors year etc.) there isn't too much of a difference between the standard BSc. and the Advanced Research degree, but I don't really see any disadvantages of doing the Research degree if you at all interested in dabbling in research.
In response to your concerns about employability/research availability, I wouldn't let it put you off if you're really passionate about physics. Again, I'm probably not the best person to be giving advice about this, but as far as I know if you're willing to put in the effort you should have a decent chance at further opportunities (especially as it's easier to network with Professor's in this degree). Even if you decide that physics research isn't for you, the transferable skills you'll learn will allow you to transition into many other fields.
Hope that's somewhat insightful