First, to have any idea what I'm talking about, you'll need to watch Douglas Cole's interview of Hans-Christian Vortisch.
Gaming Ballistics Firing Squad Welcomes Hans-Christian Vortisch
At one point, Douglas and Hans start talking about GURPS Martial Arts: Fairbairn Close Combat Systems. I'm a credited playtester on that one, not the least of which is because the work in that is based on the work Sean Punch and I did in GURPS Martial Arts. But also because I put in a lot of time on the book, reading and commenting and try to help make it the book Hans wanted it to be.
It comes in to "conflict" with GURPS Martial Arts: Technical Grappling, though, in that it says you can do things in a given time that Technical Grappling says takes longer.
Part of this is a fork in the road. FCCS is based on MA and Basic Set. TG is based on MA and Basic Set, but not on MA+Basic+FCCS. The later work takes the raw concepts in Basic Set and the expansions in MA and then says, basically, what if the grappling wasn't binary grappled/not grappled but rather a continuum of control based on an effects roll?
So naturally, some of the things in FCCS don't quite gel with that. FCCS takes real-world combinations from the system covered and then tried to tied them to the (necessarily) abstracted rules of the game and the way the game says things happen. TG changes some of those abstracted rules, expands some, and changes the way some real-world actions play out in the game. So it's hard to trace a line between them.
This isn't a bad thing, but it is a thing. GURPS is fundamentally a toolbox of a game, and it's universal, but all of the parts don't really work together perfectly. Some just aren't meant to ("A Matter of Inches" vs. "Shout It Out", for example) and some just don't because, like FCCS, it went too far down the road of using the basic rules to operate smoothly with something that changes the basic underlying assumptions of the rules it used.
Since so often a complaint is that, basically, Book A Says X, and Book B says Y, so therefore one of them is wrong or we need a Z to unify them . . . but it's not something that is ever going to be achieved, not even if we all had unlimited time to go back and iteratively change our books to reflect all the work of the others.
It all basically works together. As long as you follow the thread of abstraction and recognize that supplements that don't required Other Supplement A or B won't necessarily use the rules in Other Supplement A or B, you'll be fine.
It's universal, but all of the parts aren't meant to mesh together like legos. It's a bit more complex than all of that.