SAS, just sticking with he AFDS because that is pretty clear cut…
I seem to recall that when Bristow ordered the brand new AS332L in the early 80s, there were some changes to the wiring insisted on by Bristow, to move a few things from one bus to another. It is of course a trivial change in practice, slightly more complicated due to the weight of bureaucracy. In those days operators like Bristow had people in the procurement process who understood flying and flight safety.
Unfortunately these days procurement is done centrally and the people doing it probably only look at the manufacturers glossy brochures and the £ or $ signs. I suspect operators lack the competent people, or at least those competent people are not allowed to be involved in the procurement process.
From the manufacturer’s perspective, they have a problem in that they supply a product which could be used in numerous roles (unlike an airliner which just flies from one airport to another). So it can be tricky to match the delivered aircraft to the role, unless the manufacturer has in depth knowledge of the particular role.
But even so, basic FMEA would surely show that having an AFDS system that doesn’t work when generated power is lost (eg an engines off landing on water) is a bad design.
So who is at fault?
The manufacturer for not doing adequate FMEA relevant to the role
The procuring department of the operator for not doing due diligence on the detail and not utilising the expertise within their organisation
The pilots, for accepting the deficiency for decades without making a fuss.
So everyone is at fault. But one should bear in mind that just because everyone is at fault doesn’t mean that no-one is at fault!