A helicopter "Avoid curve" tells you when you can autorotate down and land on flat ground below you
The helicopter avoid [height/velocity] curve is not so much about the landing surface, but rather making it to the point where a flare should be begun, with enough stored energy to arrest the rate of descent to that at which you're comfortable hitting. If you're low and slow, engine quits and you're on your way down without enough height to accumulate the energy you'd like to have to fully flare with - so flare, but descent continues unarrested - until the surface.
Every airplane I have flown has the same characteristic, but the data is not published, and thus usually not trained. If you find yourself at 50 to 100 feet AGL either slow, or pulling some G, and the engine stops, you'll have to lower the nose to enter a glide. By the time you lower the nose, the ground is right there - If/when you pull the flare, you stall, and continue down.
This is of particular concern with STOL modified Cessna's particularly heavy amphibians with lots of drag. You can fly power on at 40 - 45 KIAS, but a suitable glide speed for an approach to a power off landing is nearly twice that speed. If it quits at 100 feet, and 45 KIAS, you will be at the ground before having reached 80 KIAS from which you could effectively flare.
Happily, unlike helicopters, airplanes have little reason to be flown for more than a few moments within their "avoid curve", so it's a low risk -
except for low altitude maneuvering....