I don‘t think that is what was asked for. The question is really what about the combining unit itself? And in my opinion the answer is perhaps that this unit is very simple and failure would be comparable to physical failure of the output wire or device being controlled.
I tried the search term "fault-tolerant voter circuit NASA". The voter circuit itself is a single point of failure and I guess that NASA designed that to be fault tolerant.
Figure 2 in this 1969 NASA report shows a TMR system with three voters but I'm not sure how that works