You mention an instability problem with tailless aircraft, that going much faster than the design speed makes the overall Cm negative, and the plane pitches down. You're right, as far as that goes -- as the plane speeds up and the elevator is positioned lower to keep the plane from climbing, the static margin will decrease. At some point, with enough power (or descending quickly enough) the margin will go through 0, and the plane will be unstable. Still, it shouldn't be something that sneaks up on you, it will be a gradual thing.
The same thing will happen on any airplane; you need to trim the elevator down the faster the plane goes, and at some point you'd run out of static margin. Most lightplanes have huge amounts of static margin in reserve, though. If you have enough drag and a small enough engine, you'll never get to that corner of the envelope.
I would think that tailless planes would have a pretty small speed range.