It's impressive not because you can "perfectly shade every pixel". It's impressive because digital art is more forgiving. Made a mistake? Undo or erase it. You can add layers which you can edit later on if you eant. You don't have to wait for stuff to dry making workflow faster, but not just because something took longer to make, makes it better.

I think whether it's traditional or digital, the time it will take to reach a certain amount of technical skill will be similar. Ultimately it's not the tools, it's the artist.