There is one obvious drawback: Boost.Beast requires Boost in the dependencies. There are lot of programmers why like to use Boost, but there are also people who don't want to use libraries which depend on Boost.
What benefits we can get if we will use Boost.Beast instead of http-parser (or picohttpparser)?
Yes, the dependency on boost might be critical thing.
I only saw recently word about this framework and wanted to highlight that there is boost::beast which can handle low level HTTP work (plus it has websockets in case you'd like to extend above just REST)
There are only two benefits as far as i'm aware:
1. Beast supports arbitrary method strings and has a slightly better performance(relatieve to node.js HTTP parser)
2. HTTP low level protocol could be delegated to it entirely, I'm not sure how finished framework, so it might be late to consider beast.
P,s, I'm not aware whether you already know about beast so I only wanted to highlight it.
Beast is based on Asio which you already depend on, so why is using those 2 modules from Boost such an issue?
Using beast might give better performance and better integration with asio, also I think it's better to use C++ libraries.