As a returning player, who had taken a long break since WoD, and I didn’t return to the game until now (Shadowlands), I can assure you, unlocking the Nightborne is a little bit longer than just obtaining:
You’ll need to do the following:
This will unlock the World Quests (because part of Insurrection is to complete 10 World Quests). Good news about “Uniting the Isles” is only one character, out of the entire account, being Alliance or Horde is irrelevant, has to get the factions to Friendly. When you bring a new toon to do it, it’ll be a simple pick up and turn in. If none of your characters have done the Friendly portion, you’ll have to plow through it. But, it is nice that it’s an account wide ordeal, and it’s NOT Faction restricted. Only one toon, matters, and it counts for the entire account.
This will come naturally, as one of the Quests required will give this to you.
This will unlock the quests required for “Insurrection”
Followed by “Insurrection”.
Follow every quest listed here (and, it IS A LOT of quests!):
I spent a good 13+ hours, in one day, following these guidelines, to a T, in order to unlock the Nightborne. It was tedious, but at least I got it all done 
Happy hunting!
EDIT: You will NOT need to do this quest: https://www.wowhead.com/quest=45317/fighting-on-all-fronts1
You’ll see it listed, under the “Comprehensive Suramar Guide”, but it is NOT required.