One thing that most people don't seem to catch is that this unit is just poorly designed. For a well designed used unit, it is very difficult to assemble it badly. This is the opposite of that.
Of course, then, visual inspection fails, as well.
I'd also give this unit low chances of survival in a high vibration environment; now, you might think this is not really the intended application, and you'd be right, but I'm betting there isn't a warning not to do this, so some people will.
And, I've got awful suspicions about its EMC performance; it won't really generate any noise in normal operation, but, with design like this, you wouldn't want to bet on noise on one connection not coupling to another. You'd really have to do some proper analysis and testing to be sure, but APC haven't been bothered to do it, so I don't know why I should.
You refer to this as a 'vintage' unit; any idea how old it is? It would be a bit less worrying if it were 20 years old, and a bit more if it were 2.
All in all, this wouldn't be a surprise coming from a no-name supplier, but APC? Very disappointing.