I always recomend not buying the land seperately. My background is almost exclusively constrcution loans for the past 7 years or so.
the drawback are numerous, and fairly catastrophic. the only real risk mitigator is having a great lump of cash.
1. val shortfall. if this happens after you have already purchased the land, your stuck with the land, and you have most likely already parted with 5% for the build too. At best you can arrange a new build contract at some extra cost which the valuer may like better. At worst you cant complete the project and are sued by the builder.
2. site costs more than expected. if you havent got an actual contract (not a soil test, not engineering, a CONTRACT) before you buy, you may well be surprised at how much it costs to stick your particular house on your land. Soil test may denote a particular type of slab, however when your particular house is sited, it may have to be on a certain part of the block (for instance if due to covenants you can only place the house in a partular unlevel corner of the block etc) site costs can run to several thousands of dollars, these are dollars that are essentially wasted, valuers dont account for them, and the next buyer certainly wont.
3. change of circumstances. Like OTP, leaving the purchase half finished opens you to risks your personal details, or the property markets, or the banks lending policies will change between land purchase and construction, again leaving you stuck with a land loan without income, or the expence of changing builders/lender/circumstances etc.
In my experience people only want to purchase the land first for two reasons.
They cant afford the house yet, and expect a capital gain while they 'save' a further deposit etc. It is extrememly rare to see capital gain in developer blocks in the first 3 years. Extrememly rare. (note the developer might be selling the next release at a premium, but your block's valuer wont recognise much of a capital gain)
Or, they have been convinced the market is overheating, and they dont have time to arrange a building contract before the land contract finance expires. Its a 'quick, cheap' sale. This second option is the most dangerous. The heat is on for this 'cheap' block, because everyone else who has tried to buy it has had a builder quote ginormous site costs. Steer clear.
Dont be lazy, get the build contract organised before the land finance expires.
NB, I dont mean buy a 'package' of both from either the builder or developer, I mean organise two seperate contracts off two seperate people/orgainisations timed to coincide.