Mills has done a fantastic job with Bolt but not just for the obvious reasons of top performances. Combine his marks with the fact that with few exceptions Bolt has remained relatively healthy under Mills (his recent achilles issue did not seem to be very serious as he did not miss that much comp.). As a younger athlete, it seemed he had many more injuries than he does now while putting out incredible outputs.
Also, under Mills, Bolt has continued to significantly improve (massive understatment I know) his performances. So often, it seems, the very talented junior athlete does not continue to improve or at least rarely by so much. This is clearly not the case with UB.