I never wrote a parallel for-loop in 15 years working on Firefox, because it's hard in C++, it's risky and difficult to maintain the thread-safety invariants, and it's not all that useful in most parts of the browser.
I write them quite often in Rust, because Rayon makes it super easy, there is almost no risk because the compiler checks the relevant thread-safety invariants, and I'm working on different problems where data parallelism is much more useful.
