Yup, it is probably fair to say that C++ accounts for like 50% of the complexity of Kythe at Google. Or certainly it feels like it.

And it is also worth noting that Kythe goes a bit deeper than what LSP can accomplish. In particular Kythe is built around a sort of two-layer graph, where it separates the physical code/line representation from a more abstract semantic graph. This allows us to accomplish some things that are very difficult to do in LSP.

Finally, Kythe at least internally has a big reliance on a unified build system (Blaze, or Bazel). It becomes rapidly more difficult to do when you have to hook in N different build systems up front, which is why search-based references are so appealing. Build integration is hard.