Here’s an article I found in 30s of searching (granted, I already knew Warfarin was hugely affected by CYP2D6) stating that equivalent maintenance doses of Warfarin can vary by a factor of >10 (from 0.5mg to 7mg) based on the CYP2D6 phenotype of the patient.
That is to say, if we were to actually start assessing patients’ ability to metabolize warfarin we’d have to confront the fact that a huge number of bad clinical outcomes in the recent past stemmed from not doing this.
Healthcare systems are so aware of and open about warfarin's risks that they have warfarin clinics set up to repeatedly measure patients' prothrombin times so that they can adjust dosing empirically. Further, they also provide nutritional guidance to patients taking warfarin to help them reduce diet-influenced variance in warfarin metabolism.