It’s an old saw with me, but Dave Snowdon puts it very nicely and succinctly:
Numbers are good, but they are never the whole picture. Its easy to focus on them, they give the comfort of apparent objectivity and used to support human judgement they have high utility. The problem is when they replace judgement rather than supporting it. Of course in the ordered aspects of any enterprise statistics and numbers can do a lot of the work for you, but in a complex situation they can be dangerous. Applied to ordered aspects (boundary conditions, probes and the like) they have utility, but for the system as a whole they are more problematic.