High Current Load:

Here is a table that shows what happens to the voltage a set of jumper cables can provide when you increase the current needed to start an engine without switching to a larger cable. (click for .pdf version.)

  I have had two experiences in the past when I have tried to jump start GMC Pickups with the 6.2 Diesel Engine and it didn't work.  This is part of the reason I started making my own cables.  

If the GMC has batteries that are good and just need a charge, you can eventually get them started.  But if one or both of the truck's batteries are bad, it will never work...  Unless you have jumper cables big enough to provide all the cranking power from another vehicle

This is why I use larger size cables than most other jumper cable makers.  They are even larger than the other so-called "Heavy Duty Tow-Truck Jumper Cables"

Quick Disconnect Power Rating:

The quick disconnects I use are rated at 350 AMPS continuous duty.  Of course the Pulse Current Capability is much higher. (Using 1/0 wire and connectors during a four second cranking duration they can handle almost 900 AMPS):

 [ chart is from Anderson Power Products, the quick disconnect manufacturer ]

Corrosion:

Here is a picture of a set of old winch cables that I recently replaced (click for a larger image).
 
These cables had not been abused, they were just old and over the 13 years they were in use they corroded.  Unfortunately when this happens the resistance of the cables increases and the voltage that is available for the winch motor reduces. This can leave you with a winch that is not strong enough to pull you or your buddy out of a snow bank some morning when it's -24 °F and the wind is blowing 30 miles per hour.

This is why I use a 14 ton hydraulic crimper and seal my connections with silicone sealant and heat shrink tubing.