Seems like there are two conversations here:
1) Stern shape - canoe vs. rounded vs. …
2) Boats with wide, flat aft sections vs. boats with more moderate beam aft
Seems like the early posts were #2 and the later post switched to #1. They are completely different and IMHO unrelated conversions. To me #2 is a more interesting question.
It seems like flat, wide bottoms encourage plaining and allow you to fly more sail. None of that means they sail better. Just that they are faster when sailed by professionals. If they could get an extra knot
while sacrificing 90% of their safety
they would in a heartbeat. That does not mean it is a good choice for a cruiser. Also since they increase wetted surface, they are slower unless you are using that to fly a lot of sail, which the racers do, but the cruisers not necessarily.
For certain the flat, beamy aft sections increase the space in the cockpit
and down below. That is why most (but certainly not all) modern production boats have wide back ends. Most people really do not care how well they sail. They just want two private cabins in the back.