Anything not properly maintained, neglected or abused will break over time.
XREAS ring a bell anyone? ... not hating, just saying... pressurized undercarriage systems like this over time are prone to leaking/ breaking/ or becoming a maintenance nightmare... Like anything undercarriage (a real harsh operating environnment), one needs to properly protect the parts mostly against rust which cause seized components, fittings, adjustment scews leading to premature failure..
The KDSS's most likely failure over time, from my internet research, are the adjustment set screws in the box becoming seized. (people usually only find this out too late after trying to lift a KDSS equipped vehicle to level out the truck post lift). The adjustment screws are in a metal box mounted to the frame rail behind the front tire. I think it's 2 - 5 or 10 mm adjustment screws (can't remember) that should be regularly exercised (no more than 3 turns out and in) and anti-seize applied to threads... if I owned a 4Runner with KDSS I'd also fluid film the entire system to help minimize the posisbility of leaks due to corrosion(box, fittings, hoses etc...)...
Lastly, the system has been offered as standard equipment on Toyota's flagship SUV, the Land Cruiser 200 with minimal issues since 2008. Surely it's been thru it's paces during that's platforms R&D as well as in service to date... If you are really concerned about the system I'd suggest going over to ih8mud or expedition portal and searching around.... I doubt you'll find any issues other than the exact scenario I described above. Chances are if you don't lift the vehicle, kdss will never disappoint you.