Assassin's Creed Black Flag Resynced has great news for completionists: "no achievement can be missed," and none are tied to a difficulty setting, either

I am very much not a completionist when it comes to games, but I understand the compulsion. There’s a certain ownership of a game you can only earn yourself by checking off every single little thing it has to offer, and often times, that final checklist is the list of achievements you can unlock. Unfortunately for many completionists, many of those achievements are often tied to things like difficulty settings, or activities that can only be completed if you’re connected to the internet, or worst of all, branching paths that require you to play through the game multiple times. Thankfully, Ubisoft has confirmed that Assassin’s Creed: Black Flag Resynced, the upcoming remake of one of the best pirate games of all time, won’t have any of that bologna tied to the achievements system.

As it did with Assassin’s Creed Shadows, Ubisoft published the full list of Assassin’s Creed: Black Flag Resynced achievements on Steam and, with the exception of the “hidden” ones, what you need to do to unlock them. It also confirmed three crucial details that any self-respecting completionist will be glad to know: “Available from day one, all achievements are the same across Steam, Xbox, and PlayStation.
All achievements will be unlockable no matter which difficulty level you choose,” the studio says. “Plus, no achievement can be missed during the adventure. You also won’t need a connection to earn any of the achievements.”