On the NLL, it was probably safer for the guard never to venture into the actual passenger area!!
I think the 313s (at least the FCC ones) are rock solid and rarely seem to fail. There was/is a problem with the thermostats (meaning the heating doesn't work properly) and FCC is only changing them as they fail, but besides that the trains usually work most of the time.
Obviously a failed train somewhere on the Northern City line spells disaster, but it seems to be mostly down to signal problems whenever trains get diverted, not a failed train.
Of course, the non-FCC 313s may well be in a dire state. I remember seeing just how bad the NLL trains were (and in the early 2000s, how bad the One 315s were) so maybe there are good examples and bad examples.