I'll disagree and say that it's not impossible to learn on a Cub. Howerver, your going to learn how to fix and glue your plane, which is what you'll be doing for a while when your learning.

I learned on a tail wheel model a Pilatus Porter to be exact. The "Buddy Box" is the way to learn with any airplane, but with a tail wheel aircraft, your going to learn rudder control to keep the plane centered down the runway on take off and controling your roll out on landing.

Cub's do have a tendcy to ground loop, if you don't stay ahead of the plane by aticpating your rudder imputs. Hopefully with a patient teacher, who can save your before you screw up, which you will do, the Cub can be used as a trainer.

The biggest thing I've seen with low time pilots and tail wheel airplanes, is throwing the gas all at once and then trying to recover with rudder and the plane ends up all over the flying field. The rational here that if they get the power and speed up ASAP, then can jerk if off the ground. Learn to be a smooth pilot. Always slowly apply throttle, that way the torque from the engine won't spin
you around. Rudder, Rudder, Rudder!!!!!!!!!!!!!!!!

Good luck with the Cub and if you really want to use it as a trainer, just be prepared to use some glue. I learned by dragging my tail first, which I feel made me a better pilot.