Andy-
Don't mean to beat a dead horse, there are many boats out there built using any of these methods.
As for the scarf joint, I'm not sure we're talking about the same thing so maybe a pic can help.
Here's what I'm talking about when I say scarf joint.
The top drawing shows the two pieces stacked as if you were going to create the scarf with a hand plane.
Next shows them reoriented for gluing
Last- the finished joint.
A couple things to note-
regardless of the thickness of the wood, you'll always be tapering to a feather edge. Doesn't matter if you start with 4mm or 3/4"
The finished joint is the same thickness end to end and the same thickness as the original material.
This is what makes it bend in a nice fair curve with the rest of the panel.
It also creates a smooth transition for distributing loads, no stress concentrations.- this is important in lightly built boats.
The 8:1 slope is the recommendation I've used from Gougeon Bros. and it has always worked for me.
Finally, when you bend the scarfed panel part of the glue joint is in compression and part in tension- none of it is in shear.
Compared to the "finger joint" where it would all be in shear.
Epoxy is great stuff, but my understanding is that shear is the weakest axis for the glued joint.
Here are a couple pics of the jig I built for using a router to scarf panels.
If your panels are very wide you'll need to adjust.
I use a piece of plywood as the sled base for the router to slide back and forth.
Just make sure its long enough to support both ends at full reach.