It doesn't really matter how many dependencies Boost.Thread adds. What matters is how expensive is. Personally, I would have no problem depending on it, if I really needed this component.

One problem here is that Thread is effectively unmaintained. In general, I'm not a huge fan of depending on libraries like this in my own tests because Unordered has a very strict set of flags and many Boost libraries won't clear the same bar. I remember in the past having a test dependency on one of the bigger Boost libraries and it wasn't exactly painless. I can't remember now if it was Container or Filesystem. Boost.Compat has utility in that it's essentially just a collection of substitutes for all the goodies the committee added that could've been in C++11. This benefits both authors and users massively, instead of purporting that anyone should rely on an unmaintained lib. - Christian