Dexcool is like many other long-life coolants that are based on organic acid technology. The formulation is unique, but the additives can be found in many other coolants. The issue with Dexcool is that when a cooling system developed a leak, which introduced air into the system, fumes from the dexcool + oxygen + heat formed a very corrosive mixture that would attack any steel parts inside the cooling system. That is the source of the brown sludge many people complained about.
Properly maintained, a system containing dexcool will work fine for a long time.
The problem with modern GM cooling systems isn't dexcool, it is the use of plastic parts in the cooling systems. the plastic will become brittle and break over time.
The intake manifold gaskets in many GM cars were basically o-rings that were supported by a plastic body. The plastic degraded over time and cracked, allowing the o-rings to squirm around and eventually let coolant seep past them. Many newer gasket designs use metal bodies to hold the o-rings instead. Or, the intake manifolds themselves are designed with grooves to hold the o-rings, such as in LS motors.