I haven't dived deep enough to figure out if integration with existing C/C++/Fortran libraries works out-of-the-box, or whether you have to setup bindings a la Python. If it works out-of-the-box, my question is moot, especially if two-way interoperability with Nim code is seamless.
You have to set up bindings but it is as simple as it could possibly be to do by hand, and there is a c2nim program to help.