It might make more sense for you to run a unikernel but it may also be important to you to use your existing tooling / monitoring infrastructure to administer your server. For example, being able to ssh into a server when things go wrong can make things a lot easier.
There isn't anything inherently different about a WebAssembly-based software sandbox from previous software sandboxes like the ones you mentioned. It maintains a reasonable level of protection for additional benefits. If those benefits aren't worth it for your use case then you don't need to take the additional potential risk. We don't live in a computing monoculture, what's risky for you may not be risky for me, and what's valuable to you may not be valuable to me.
It's worth noting that the code that will be run under Wasmjit will in general be more trusted than code you run in a browser while browsing the Web. For those who only run trusted code on their servers, Wasmjit incurs no additional risk and provides potentially significant benefit.
