Kustomize (the kustomize CLI / kubectl kustomize) is a manifest builder: it composes bases and overlays, applies patches, and emits YAML. It does not apply anything, and it has no notion of ordering, readiness, or reconciliation — that’s kubectl apply’s job, and kubectl applies everything at once.

What Kustomize gives you

• Overlay composition, strategic-merge and JSON6902 patches, variable replacement, generators.
• A pure transformation: in goes a kustomization, out come manifests.

Where StageSet differs

• It delivers, not just builds. Kustomize stops at YAML. StageSet applies it, waits for health, prunes what’s gone, and keeps doing so.
• Ordering and gates. kubectl apply -k has no stages and no gates. StageSet sequences stages and runs actions between them.
• Continuous reconciliation and drift correction, versus a one-shot apply.

Using them together

StageSet includes the parts of Kustomize you reach for at delivery time: a stage has path, patches, and postBuild substitution (stages and sources). So you can keep authoring with Kustomize overlays and let a stage apply the right overlay, patched and substituted — then add the ordering, gating, and reconciliation Kustomize alone doesn’t offer.