Iwo Mergler wrote:

It's possible to use the effect in an but
there is no concept of an angle of attack and the
L/D ratio is quite bad at low RPM.
...

The first thing (no concept of an angle of attack) is a good
reason that no airplane may have success using this effect.
An essential condition for an heavier than air aircraft to be
controlable is that its lift coefficient (CL) increases with the
angle of attack. So that if anything disturbs it from its previous
path by an upward deviation the angle of attack is decreased, and so
the CL and the lift (because the other factor, the speed, needs time
to change due to inertia), so the weight becomes higher than the
lift and this gets the aircraft back to its initial path, similar
things in case of a downward deviation. This is why airplanes and
gliders become uncontrolable at stall angle of attack, not by
lack of lift, the CL is then at its maximum, but due to the impossibility
of increasing it further by increasing the angle of attack.