CUE improves in Jsonnet in primarily two areas, I think: Making composition better (it's order-independent and therefore consistent), and adding schemas.
Both Jsonnet and CUE have their origin in GCL internally at Google. Jsonnet is basically GCL, as I understand it. But CUE is a whole new thing.
jsonnet is basically GCL, but after like five failed attempts to rewrite, replace, or fix GCL, which included creating a formal semantics of GCL pointing out all the problems. I believe there was a final successful attempt to fix it.
