Seems to me that lifting tails are, and have been, illegal for
long time. The regs call for the aircraft to automatically settle into
a glide if the power should fail, to prevent stalling. A lifting tail
just won't do this. As the airplane slows it will drop, raising the
nose, and the airplane will stall, and almost certainly enter an
unrecoverable spin. If the pilot does manage to establish a glide, the
nose will drop further as glide speed increases, opposite to what we
know in our airplanes, and totally unstable.

A good reference for this subject is "See How it Flies". It's not true that a
tail with positive lift will necessarily lead to instability:

"The biggest contribution to angle of attack stability is decalage. The thing
in back flies at a lower angle of attack than the thing in front. The thing in
back may, but need not, fly at a negative angle of attack."
http://www.av8n.com/how/htm/aoastab....oastab-summary

The pitch stability discussion is at:
http://www.av8n.com/how/htm/aoastab....ch-equilibrium