Isn’t international trade another component of this? Won’t all the untaxed robots overseas, or even just offshore end up producing all the goods? Sure, a tax could get applied on imports as well but besides protectionism it would be a complicated mire. In either scenario whomever is producing goods with taxed robots is at a competitive disadvantage.
I also think your point as to what would be defined as a whole robot gets over simplified when describing humanoid robots, but it is well founded. It would drive innovation to avoid whatever determined taxation. That doesn’t sound like the best basis for progress.