Are amp still doing that?
To my knowledge, yes (and St George). I haven't tested this in a while and would run this scenario via my lender contacts before submitting.
In terms of cons, Jess has outlined it pretty well.
I'm not sure about the comment around whether you can grow a very large portfolio. I guess it depends on your definition of very large - but the strategy of joint purchases can also assist in building larger portfolios as joint ownership often opens up opportunities to reap significant rewards that would otherwise not be possible.
As with any investing strategy, you've got to have the right frameworks in place and strategy, but no reason why you can amass serious wealth by doing this.
From a finance angle, you can borrow and grow a sizeable portfolio. It would depend on normal servicing policies and then taking advantage of lender niches. St George are pretty conservative, so you may struggle with them. AMP on the other hand have a very generous serviceability calculator, can go pretty far with them usually.
As you grow your portfolio to extreme limits and start relying on the 'last few lenders' (Macquarie/others that service on actual repayments and no rent reliance policies), you may need to address the joint purchases. Nonetheless, you should definitely be able to grow a larger portfolio without this holding you back for a while (250k income!) - just need to choose the right lenders and have a tailored plan.
You might be able to get the NRAS incentive working for your borrowing power too with FirstMac (you'll need another security with them too though). Having 11k after tax added to your income is a very powerful addition to your borrowing power. They also take debts at actual repayments, so utilising them as part of your lending plan may extend your journey.