FWIW I have seen more than one shimano hub that was declared 'broken' but in fact the connection to the hub was bad at the plug. So that connection design also (like many things elec-trick-eral, including spade connectors) looks simpler than it really is, which I guess is what I was angling at in relation to other connectors too.
In the case of the shimano connectors a combination of
a) the wires in the plug being a bit thin or slightly misplaced
b) even slight corrosion on the wires or the hub contacts and
c) slight deformation of the hub contacts
can result in an arrangement that looks perfectly normal but actually gives no contact at all, or an intermittent one.
On one bike I have, the headlamp wire came to me a bit short and a bit skinny for my liking. I spliced in an extra length of wire near the hub, and I used wire with about x3 cross sectional area in the copper, mainly because this gives a more consistent contact with the shimano plug design.
I've also run shimano plugs using just the inner part before now, because the outer part doesn't always want to come off when you need it to, harbours water, and hinders you from seeing if the wires are really in the right place or not.
There is a common problem with folk's experience of bike parts which is 'the fallacy of perfection'. To the casual observer everything is 'perfect' until they see it go wrong for themselves: Something can be a pretty hopeless design such that it only works 90% of the time (no matter what you do with it), yet this will still lead to nine out of ten people reporting a positive experience with it. Equally the design can be OK if installed and used in the way intended, yet a negative experience which is due to a true design failure is not easy to discriminate from one that is otherwise attributable, eg not reading the instructions or just assuming that something is 'very simple' (it rarely is, IME).
cheers