There are of course times when a commercially available component just won't do, but such a decision should be made only after commercially available components are considered very carefully. Design for function. Sounds obvious, but is too often overlooked. I reduced an instrument design from over 90 parts to 19 by asking what was the real function of the device, and the new design passed a much tougher vibration specification than the former. It is much cheaper to correct designs through analysis than after hardware is built.
Analysis is hard, exacting work.
Most engineers do not have sufficient mathematical background to do such work and thus blunder along from one inadequate design to another. These rules of engineering design have been clarified to the author as a result of many years of design practice, observing the design practice of others, and teaching of engineering design. The overriding lesson is that a good designer must know himself, understand his motivations, and appreciate the fundamental reason someone else is willing to pay his salary. The fundamental objective of engineering is to create wealth out of thin air, i.e., out of ideas. A good engineering manager will to the best of his ability and resources provide an environment that will bring out the creativity of his engineers, but the good designer will not wait for such an environment to be created.