Kant's ethical system did suffer from rigidity mainly do to the fact that he didn't necessarily have ethical principles in hierarchies, so as a result truth telling and the value of life principle were on equal terms of significance. W.D. Ross improved on Kant's ethical system by introducing Prima facie duties which set up ethical principles in levels of importance, so that the individual wouldn't act unethically by lying to the murderer chasing a victim down. In regards to Kant, I've never been a fan of his Practical Imperative, sure superficially it makes sense but in some scenarios it results in more harm than good.