I'm not an expert, but I'll try my best to compare it to docker:

You can think of nsjail as a lightweight frontend for kernel isolation features like namespaces, sys call filtering, and the like.

Docker is also a frontend for some of those kernel features, but its original goal was not security isolation, but rather isolation for the sake of reproducibility. This isn't to say that docker isn't secure, they did add those features eventually, but they are less intuitive to change and mess with, and you have some added complexities. Whereas nsjail stays as close to the system as possible. As far as I'm aware, there's no concept of an image, and its not necessary for every app to have an entire OS user land with it (although you can if you want to).

If your goal is security, docker's defaults are reasonably good, but also made to not get in the way of most applications. This might be good enough for many. However, docker's security is more difficult to customize and less straightforward if you need to change it.

If your goal is security, and more so than docker's defaults, nsjail gets you there muchhh more easily. Whether nsjail has more security features than docker, I don't know.