Is it really a time sink? You have to have a build system anyway to link together your project itself, assuming it's bigger than a single-file hello world. One extra line in your build system should be the least time consuming part of adding a new foreign dependency--you still have to vet the code for security and figure out how to use it.

Unless you're using C++20 modules, you also have to deal with possibly including the header multiple times (slowing down builds), namespacing, macros potentially defined by the header, or a bunch of external/internal linkage edge cases. You only ever find out about these problems once it's too late to remove that library for a different one.

It depends. But if that header only library is now in your hands - good luck modifying it without paying the penalty of recompiling anywhere else it's used.

Also, now have to prefix the hell out of it, as even anonymous namespace won't work. And not only that.

It feels like more being lazy not to learn the proper way of doing it.