What is unreliable is ignoring the fact that a scientific, falsifiable, prediction-generating version of "fitness" will involve laws, and that the resulting law-constrained stochastic search does not have the philosophical content needed to make Darwinism into an atheistic argument-from-no-design.
Retracting my rhetorical concession, I would point out that which is created in the image and likeness of God is also in the same account marred by an action which separates him (us) from the ultimate knower. The perfectly built telescope with a smudged mirror will show false images, so the disordered senses, reason and noetic faculty of Man is no longer necessarily a reliable guide to truth.
You really can't say what the laws defining fitness, the adaptive landscape, etc, will be like, since we don't know what they are.
IMO Darwin's insight that the environment treats organisms rather like an animal breeder does, and that this plus lots of time suffices to explain the diversity of life, will stand.