Um, yeah, you won't be able to model artists who don't follow a model (especially done so deliberately). As you say that is true of humans or computers alike. But it's not the problem anyone cares about and not what the parent comment intended.
Certainly a well trained model will be able to have incredible accuracy just with vocals alone. It will be able to identify Lady Gaga regardless of whether she is singing a new art pop track or old standard with Tony Bennett.
We could have a debate about the consistency of Gaga or Taylor Swift and profit a motive (and we could go all the way back to composers of the classical period with this).
I could also point you to Diplo who, as a "producer" is responsible for diverse sounds with his name directly on them and then side projects Like Major Lazer or MIA's paper planes that have his hallmarks but aren't "musicaly" linked. How about the collected work of Richard D. James, I'm no so sure that all the parts fit together outside the whole of them.
Stuart Copland was the drummer for the police, a very distinct and POP sound. Are we going to be able to use ML to take those works and correlate them to his Film scores? How about his opera? Dave Grohl, Phil Colins, Sheila E, more drummers who became singers, what is the context for ML finding those connections (or people).
John Cages 4'33 is gonna be an interesting dilemma.
DO you think the player piano black hole sun, and C.R.E.A.M cover from Westworld are picked up as stylized choices by Ramin Djawadi, and would it link those to the sound track of Game of Thrones?
Even with all the details it's sometimes hard to believe what talented people can do and how diverse their output can be!
Certainly a well trained model will be able to have incredible accuracy just with vocals alone. It will be able to identify Lady Gaga regardless of whether she is singing a new art pop track or old standard with Tony Bennett.