Fault trees

I note that the recent Falcon 9 launch failure is being investigated with a fault tree analysis.  When I was working in quality assurance in 1968-1970 fault trees were one of our primary tools but even then we felt they suffered from some shortcomings. It was hard to add numerics to them.  Furthermore, you might describe the binary success or failure of some part or event but how did you describe  a partial failure or a partial occurrence?  Modern Bayesian networks seem to offer some advantages in describing causal sequences.