When it comes to pushing against fluids (such as water and air) you gotta get speed, demon speed! Speed is what we need! We need greasy-fast speed! It's rather apparent that slow-moving objects tend to meet less air-resistance than faster ones. Birds use said resistance to push against the air and go upwards. Well, there's more to it but that's the basic idea.
For powered fliers (which are more-or-less the bane of my existence), their wingtips travel the longest distance under the same angular velocity, i.e: they're the fastest part of the wing and thus have the most potential to generate force...
Yet, out of all the wing shapes I could find, only Alduin's (see pic related) seem to even attempt to get broader towards the tip.
While one explanation could be that such adaptation never evolved, or had the chance to evolve; it could also have required stronger bones and skin to withstand the forces involved.
Or alternatively, I missed something that would have revealed that this wing-design was terribly ineffective.
I honestly don't know. I already justified most of my dragons' characteristics with biogenic super materials, I could wave my hands and say that dragon wings can take a lot of stress without being heavier, but would that make this wing design superior to others at force-production during flapping flight?