The D40's downside on rough stuff is its long wheelbase. Park it beside a Land Rover or a D22 - they have a shorter wheelbase, which improves their ramp-over - the height of an obstacle that the vehicle can clear climbing over it. This includes climbing an incline and getting the vehicle onto the top.
The front diff isn't the strongest either and suffers badly with sudden shock loads for example if you have masses of wheelspin followed by solid grip, like when a wheel is in the air and suddenly hits the ground hard.
Lockers are available for the rear. You can raise these things quite a bit if you're willing to get the parts to do that (upper control arms, extended brake lines & ABS cables etc). Diff & gearbox breathers should all be extended - ARB sell these.
It depends what you mean, too, by "real 4WD". Equipped with a decent compressor and tyre deflator, you will go to the beach, let your tyres down to 20psi each and cruise happily along the sand, straight past the moron in his Hilux that insisted his "real 4WD" didn't need to air down. You can fit a wading bra to the front of your bullbar and cross straight through the river, water lapping your windscreen, and continue past the fool in his Vitara that has to patch his radiator to repair where the radiator was pushed against his cooling fan.
What is a "real 4WD"? In my mind, it's any vehicle that has all 4 wheels driven by someone who is competent in doing so. We've had D40s at Oodnadatta, crossing the Tanami, Canning Stock Route, Gibbs River Rd, Birdsville Track and more. They are quite capable of doing normal 4WD things.