I started a new game (fresh file, not new game+) of AC6, and I beat the chapter 1 boss on my first try with a tank + grenade launchers build and then on my first try with a reverse joint legs + burst rifles build, neither of which I had used against it before, so I think I'm allowed to say that at least this boss really is more of a skill check than a build check. I think it's just that different things in the game are initially easier than others to keep track of and do without having to think about them for different people, and that influences which builds will initially be more effective for you.
Also, I'm so much better at this game than I was a few days ago, wow. The power of being able to dodge and keep track of your cooldowns at the same time...
And now I've also killed the chapter 2 boss with a few different builds. Screenshot is of a tragic double KO, but after that I won and even had a repair kit left over. I got the chapter 3 boss on my second try and the chapter 4 boss on my 10th or so the first time through, so I'm going to say I'm competent at this game now. Nice.