Assassin's creed Ezio collection 100% help

Me and my brother are racing to 100% all assassin's creed games, we're onto Ezio's story and I have got all achievements for AC2.

I'm just about to move onto brotherhood but I'm beginning to see things we're people are explaining that some achievements don't track fully or never pop. Has anybody else had issues with this? Or any other advice on bits of junk that may come with the achievements in the Ezio trilogy.

Just a heads up I've completed all games in the past just never went for achievements so spoilers are fine if necessary. We're playing the games as the collection and not individually.