The whole system, technically. Really depends on your approach. I'm sticking with Reid vapor issues versus material compatibility. A (formerly) typical mechanical engine driven pump with a booster requires more consideration of the liquid system than a system that relies on electric pumps, IMO. You're asking a lot of a system that requires suction to a relatively high and hot location. The boost pump would usually be mounted very low and probably in a cooler location. For this configuration, there is a possibility for recovery. The higher criticality/lower probability part to me would be the fuel system through the boost pump. No back-up or almost no chance for recovery if you get vapor there. There's not a single correct answer here, IMO. Whatever approach is utilized, the entire fuel system from the tanks through the PP, returns, and associated systems (power redundancy) has to be considered. Sorry for being vague but while the answer/consideration isn't very deep, it is broad.
I'm sure others will weigh in and many probably disagree. I'll state again, beware of anecdotal evidence that any particular approach is OK. This is a good topic to exercise any skeptical nature you might have. I have that in spades but then again, there's something wrong with me.