[wat10000]

It's not like you have a choice. Thread safety doesn't compose. A function that only uses thread-safe constructs may not itself be thread safe. This means that using concurrent data structures isn't any sort of guarantee, and doesn't save you from having to think about concurrency. It will prevent you from running into certain kinds of bugs, and that may be valuable, but you'll still have to do your own work on top.

If you're doing that anyway, it tends to be easier and more reliable to forget about thread safety at the bottom, and instead design your program so that thread safety happens in larger blocks, and you have big chunks of code that are effectively single-threaded and don't have to worry about it.