My point was that once you have practiced on your hand, you are used to that hand, however when dealing with plastic hands, you do not experience what to do if someone has a lot od natural oil and how you deal with that whether you are doing shellac, gel, or enhancements. If doing enhancements, how can the 'teacher' check your prep and how you've attached the enhancement to the natural nail? What positioning you've used? How far down to you file to do a re-balance, without putting pressure on the nail, or damaging the natural nail? If you're doing gels, how can they check how far you've filed the natural nail? This is only a few of the points I can see problems teaching online.
In answer to practicing on friends....that's a great idea, but I would say only when you are absolutely sure, with hand on heart, you will not cause them pain or damage. Otherwise you are liable to give them short or even ling term damage to their nails.