Recently I was touting the virtues of NodeJS and the person I was speaking to said "in reality how much code can you share across the stack?" I had to point out that this was not a virtue that I extolled and I've never thought of this to be a benefit of JS full stack.
In my opinion, shared code across the stack is a bonus and not a benefit. The primary benefit comes from the synergies and efficiencies in working in a single language through the stack. This means that if someone who traditionally worked on the front end has to dive in and do some work on the back end they are not going to be shocked by a different syntax and instead will be somewhat comfortable with what they see.