: Very cool, but it seems like it would pinch the top of the wood.
: Does it leave a flat smooth top at the joint?

It's surprisingly smooth-cutting on WRC and yellow cedar.
I just use mine for rough trimming and lopping off those troublesome overhanging strips; I still grab the saw & block plane when making scarf or butt joints.
It's terrific for reducing those 1-2' 'waste' strips to epoxy stir sticks in a hurry. ;-)