And happily, since this is a front-end rather than an incompatible fork or something, you don't have to choose!
If you wanted you could totally use this most of the time and then write custom Nix expressions only if you felt like that would serve you better. For many use cases, this might be sufficient.
If you are comfortable with nix itself, then it allows you to do a lot more than devbox does, but at the cost of extra complexity.
Depends on what level of abstraction you're looking for.