Boost PropertyTree is not a json library (but it uses internally one). It drops tons of functionality from the JSON format. I also think it treats integers as strings. It's 20 times slower than the most used json libraries.
Its implementation looks very oldish. It's basically legacy code in its utmost definition.
Also, with the new c++ standards, many boost functionality can be replaced quite easily. Boost remains a bloatware for small projects, so I try to avoid it when possible.
As for other alternatives, these two are header-only (easily to implement/copy-paste on Extern/include/ or from "pkg install") and much more beautiful: