For what it's worth I actually think you older code is more readable, here is how I would have improved it:
const followers = (nodes) => {
return _(nodes)
.flatMap( node => node.followers)
.filter( follower => follower.email)
.map( follower => follower.email)
.uniq()
}
I think some would disagree with this, but I personally favour readability in code and I think your old code is easier to reason about. As a bonus, the time required to complete this also faster.