I think it's worth remembering that fender washers are not a bad option.
It depends a lot on the geometry involved but I think the plate is best in these cases:
1) Hardware is really spread out, or unevenly spread out
2) You intend to build the plate significantly larger than the hardware
3) You can't fit large circular washers because of limited space outwards.
However when these arn't true, not only do I think washers can be "fine" I think they can be better than the types of plates you often see. For one, a circular washer is a pretty good way to transmit force and does so nice and evenly.
Second, consider the scenario I just encountered - a circular piece of hardware, roughly 3" in diameter with a 3" circular backing plate. The problem with the plate is that it's only transmitting force in a 3" circle. Fender washers however, comparatively, spread the force outward (if there is space to do so). If I install fender washers and then draw a circle around them I'll have something with a diameter of 4" or more. Yes there are some gaps in this circle but the fiberglass will effectively bridge some of these gaps and overall if I replace this compact backing plate with large fender washers I expect it to be a net positive.
I think a properly designed backing plate always can be better than fender washers, but not necessarily significantly, and perhaps not worth the cost.
If you don't take my word for it I believe I've seen Mainsail say something similar - that fender washers with a sound deck are perfectly fine.
G10 backing plates
Note: McMaster.com sells 316 fender washers for 1/4" hardware up to 3" in diameter - that's huge.