Elon Musk has tweeted that "Cause of hard rocket landing confirmed as due to slower than expected throttle valve response." I am curious about what data SpaceX has available to support this kind of diagnosis. Ideally you would put sensors on every single component of the rocket and transmit measurements continuously, but that would presumably involve a lot of additional cabling and electronics, which might or might not be a problem. Or maybe the sensors don't add much complexity beyond what you need for the control systems anyway? I also have no idea how much data one can reliably transmit. Is any of this documented publicly?
SpaceX has informally noted that they add more and more sensors as issues arise. In sort of a horses out of the barn mode, after the fact. By this point they have a fair amount of sensors.
They clearly feel confident enough from the data collected to be able to make assessments of the various failures in the past. (Landing failures, since they are doing pretty good on launches).