NCM ancillary plugins that interacted with e.g. libclang were more what I was referring to, but I'd hoped the average reader could deduce that.
Those features of LSP I do not use, namely because LSP is a buggy mess. Some files have wild syntax errors reported for them that don't exist when compiling, but not others. Half the time, the auto-completion simply doesn't do anything. Nothing pops up, nothing auto-completes, etc.
And now, just a few weeks after switching over, without changing a thing about my config, the arrow keys for selection have completely stopped working inexplicably.
I feel like if your comments' utility to the conversation would be greatly improved by prefacing them with something like "in my experience with clangd, treesitter and neovim".
I say this because my experience with LSPs is the polar opposite of yours[0]. While I believe there's value to sharing our personal experiences, I think we have to be careful not to present our personal experience (especially with (neo)vim setups, which tend to vary wildly from one to the next) as representative of the whole ecosystem.
[0] I also have nothing but praise for most of the LSPs I've used (clangd, typescript-language-server, rust-analyzer, clangd, pyright, css-language-server, sumneko/lua-language-server, of the top of my head).
It's also important to note that LSPs almost always provide you more than auto completion (for example, go to definition, go to implementation, find references).
Do let me know if you have more questions, neovim and LSPs are my "daily driver" as a dev, so to speak.
[0] https://github.com/ncm2/ncm2