Its age AND the way the car has been driven/looked after. Unfortunately most GTR's have hard lives and are regularly pushed to their absolute limits. A car can only take so much of this until components begin to fail. Many cheap 32 GTR's will be the worse examples that are basically on their last legs...
Rebuilt engines can still go pop too, depends who rebuilt it, how it was driven afterwards, if they modified the engine to counter the oil circulation problem, if they rebuilt it with forged internals (much stronger)... But if a car has had a rebuild it is more likely to have had such a rebuild out of necessity so the engine has been pushed HARD and eventually died. That means that everything else in car was pushed, like diff, g/box, suspension, chassis itself... So those other components are more likely to be very tired also..
A few things to think about.