Traction+ and the ELD (on a 4x4) can be a bit slow and dimwitted, but it can help with loss of traction if used carefully and you work with it rather than pushing the button and hoping!
Basically, the car needs an open differential as when you corner one wheel needs to travel further than the other on the same axle.
This is great in a car park or out on the open road, but as soon as one wheel losses traction due to slippage, the other on that axle will not power as the open diff will not send power it, so you stop dead when one wheel losses traction.
What ELD (all four wheels) and Traction+ (front 2 wheels) does is use the ESP and ABS systems to brake the wheel that has lost traction and is slipping on that axle, braking this wheel causes the diff to power the other wheel on that axle which would otherwise not get powered due to the open diff arrangement.
It's very similar to ESP, but works tighter (not as much slip before it kicks in) and because of this, at a lower speed.
Now you can start to see some drawbacks, which is when all wheels on that axle lose traction
(not so much a problem with 4x4 as there are 4 chances of one wheel gripping)
It starts working and starts braking the slipping wheel, but you can start to lose momentum due to this and grind to a halt or sink and dig in as the cars weight overcome the momentum, so it's best to still be gentle with the throttle.
It needs to detect slip to work, which is a little counter productive, in slippy conditions to really do need to avoid this if possible, again it can effect momentum, slowing you up and on inclinces you don't want the car's weight to overtake momentum.
Another drawback is using the system when the wheels are unlikely to slip (ie on tarmac)
This can stain the transmission as it fights the wheels that naturally need to slip when cornering.