The bottom line is that if Maxi-Cash goes bust (completely or even very selectively, it looks like) or misbehaves, you lose. This is not a sovereign or quasi-sovereign bond, and it hasn't even been assessed by a bond ratings agency. There's no SDIC backstop, no deposit insurance.
Maxi-Cash is telling you that they're taking your money and betting on overseas real estate, so far exclusively in Australia. Thus their investments have both mortgage default and foreign currency exchange (SGD-AUD) risks. They will keep most of the upside because they must to try to avoid default with you, plus they want a profit.
It's a bit hard to tell, but it looks like Maxi-Cash reserves the right to pass along the underlying real estate project risk to you on a selective basis. That is, it looks like each project has a standalone risk profile. So when you contract for one project, if that particular project goes bust and Maxi-Cash cannot keep you whole from that one project's prior cashflow/upside, you lose. They haven't indicated whether they pool the risk across projects, so you should assume they don't. But even if they did pool risk across projects it'd be exclusively Australian risk (AUD and the Australian real estate market) since they don't have any projects anywhere else yet. If either the Australian dollar or the Australian real estate market crashes (or both), Maxi-Cash is in trouble and therefore you're in trouble.
On top of all that, what's the interest rate they're offering? They don't really say, but the example on their home page is a little below 3% on a 12 month hold. Does that mean you're assuming substantial risk? Of course it does. "Just look at the yield" versus 12 month government bond rates (SSBs for example).
You can already bet directly on overseas real estate via REITs and REIT index funds. If you want to take real estate mortgage default and foreign currency exchange risks, you can already do that on your own and capture all of whatever upside is available.
So...do you want to lend someone your Singapore dollars to go buy an Australian REIT that holds one building somewhere in Melbourne (for example)? At ~2.9% (for example)?