I think LLVM is a perfect example of what happens when it's too complicated: it's slow, it's bug-ridden when you stray away from the beaten path (e.g. Rust hits bugs in LLVM like this one https://www.reddit.com/r/rust/comments/l4roqk/a_fix_for_the_... ), and it's really hard to use and understand.

It's obviously not useless because of that, but it's a great example of what happens when you cannot fully control the implementation complexity