From what I've head: these models are very accurate in estimating the mean global temperature difference even for long periods of time.
While the global trend is clear, they cannot estimate local developments very accurately - these are subject to chaos, and local changes.
Note that the authors do not try to predict any local effects.
The alternative could instead be a system with open standards that many vendors implement, and only relying on standardized behavior.
This works to some extent with for example SQL or C, where you can migrate from one DB or compiler to another with limited effort.