Adaptive systems... more reliable or not?

Would the adaptive systems or the systems that can adapt themselves to fluctuating environmental conditions actually bring the reliability? Or would the reliability is compromised in such a system?
One can argue that the internal structure or the behavioral patterns of the system are tend to change more in an adaptive system. Thus the initial reliability index can be changed with the environment changes. And also prone to be attacks/failures that never imagined before.
But again adaptiveness or the flexibility can also be a sign of a reliable system. Like a grass bending to the wind. Or take us, the humans beings for example. We adapt to the environment and thus survived so far in the world(Until we try to adapt the environment to suit us ;-) )
So how far we can go with adaptiveness in a system? What are the areas that we need to consider in designing adaptive systems? How to assess/predict the reliability of such systems?