He isn't in the minority everyone I know who owns a xb4 and have raced with I have seen them replace diff gears at a race. The buggy should have never came with composite gears in the first place. My gears haven't came apart because I put metals ones in before I started racing. There is a problem and it's composite gears inside a diff in an off road buggy.
It is in the minority unless you think there are only about 12 people running the XB4. There's Probably more like 12,000, I guess in the US it must feel like you are the only country in the world. At my local track there are 5-6 running the xb4, and we've not ever broke a diff. It's not even the composite gears inside the diff, check your info.