Factory connecting rods are often underestimated when doing a tough engine rebuild. In reality, most well known engine manufacturers like Nissan, Toyota, Mazda and Mitsubishi make very high quality products for their high end performance vehicles.
99% of the time, when an engine fails, is pulled down and found to have mangled or twisted/bent a connecting rod/s, the cause of conrod failure is an after effect of the inital cause. When you see a conrod hanging out of an engine block, 9 times out of 10 it is not the fault of the conrod.
Extreme pressure is required to bend any metal component and the conrod is the link between the piston and crank that absorbs all of that energy. All it takes is one hard bout of detonation to bring it all to its knees. Exerting extreme pressure on to the rod, thus forcing the oil film out from between the crank and conrod bearing. This causes metal to metal contact and bruises the surface of the bearing. This is the moment where engine failure begins.
Many high quality oils, like Royal Purple, can withstand these extreme pressures due to their high film strength but often, due to ignorance by the owner, the type of oil used is usually not taken much into consideration. Most of the time it is a quick trip online to a favourite forum or down to the local Supercheap/Autobarn to listen to the advice of keyboard experts or a 15yo boy that knows as much about oil as a 15yo should....practically nothing.
Not all engine failures can be attributed to detonation, though, but it is one of the biggets killers. Oil starvation or contamination, metal fatigue or incorrect assembly are other possibilities which occur.
A majority of the time, the factory conrod is able to withstand 2, 3 or even 4 times their original power expectation but usually certain steps need to be taken to ensure their reliability. The only thing that lets down these items down is usually the conrod bolts/studs which cannot handle the kind of forces and pressure exerted on them at power levels 4 times their original intention. This is why it's a good idea to replace the factory items with something like an ARP stud with a pressure rating of 200,000psi or over.

****CAUTION****

Remember, when replacing the conrod bolts/studs that the conrod big end tunnel MUST BE RESIZED.
This is very important due to the fact that the tunnel can be distorted once the new bolts are torqued to specification.
Factory bolts will often have a torque level of around 40-50nm which is where they usually stretch to around 5-6thou over their original length. Ofcourse, these bolts will return to their normal length when removed from the conrod.
ARP rod bolts will often require torque levels of 60-80nm . They also stretch by around 6thou at these torque levels but the effect of the extra strength/pressure on the conrod cap usually tends to shrink the tunnel by small amounts like 0.0001 to 0.0005. It also means that there will be no gaurantee that the tunnel will be round anymore.

There have been many advancements in conrod technology and bearing technology, that nowadays you have to really consider whether aftermarket conrods are even worthwhile.
Well the answer is - YES - they are worthwhile and they DO have their place. The difference between a factory conrod and a forged aftermarket conrod is simple.....If you want to make XYZ hp and it is well documented that people are getting around with those kinds of power levels on those factory conrods, then they will definitely do the job. However, if you want to make that kind of power and you don't want to rely SOLELY on the skill of your tuner, then the forged connecting rod is the way you want to go. 
Every metal part has it's breaking limit and unfortunately, at some point, somethings gotta give!

Back to causes of engine failure, the factory conrod is still able to hold it's own when everything is running right but it is unlikely to be able to be pulled out of a failed engine and reused. The forged conrod can withstand higher pressures and is made in a way that is designed to withstand these pressures all day every day.
Most people will change to forged conrods due to the fact they are usually lighter also. This means that reciprocating mass is lower and, therefore, the engine will rev much more cleanly and quickly.

These days, with the low cost of parts manufacturing from Taiwan and China, the United states and UK, high quality aftermarket conrods can be had for as little as $400 for 4 cylinder engines and $600-$700 for 6 cylinder engines. You have to ask yourself - Isn't it a small price to pay for a revving advantage and reliability peace of mind?
Yes it is.
Yes the factory conrod will do a great job
Yes the factory conrod is a cheaper option that has been proven to work
No the factory conrod is not as strong as it's forged brother
No the factory conrod is not as light as it's forged brother

So when you are next considering your parts list for the engine rebuild. Sometimes it is worth thinking about more than just 'will it do the job?'.