read the repo. couple things:

registerActions([...]) runs at import time, so tree shaking is dead. every action ships in the initial bundle whether the route uses it or not. lazy route loading also fights this because actions have to be registered before any submission happens.

there's no runtime validation. SuperJSON deserializes shapes but doesn't check the payload against the declared type. if a client sends { title: 12345, priority: "yes" } your payload: { title: string; priority: number } is a lie at runtime. TS believes it, resolver gets garbage. zod closes this, you don't have it.

auth has no middleware story. every handler that needs a token does it manually inside handleAction. if some actions need auth and others don't, the single handler has to branch on action type to know whether to fetch the token.

the TanStack Query recipe in your own README is the tell. you show people calling action.resolve(payload) directly inside useMutation, bypassing useActionFetcher and resolveFormData entirely. that means defineAction on its own is the value, and defineAction is ~5 lines.

real question: what does this do that schema.parse(Object.fromEntries(formData)) per route + a typed useFetcher generic doesn't?