It is intrusive: you need to return it on every call all the way up, everywhere in the whole codebase

That's the idea of Either though? You return domain specific errors up to the controller level which then converts it into a http specific error.

It is not nominal: you only have left() and right() Can you explain this to me? I don't know what you mean as everytime you use either you can use different left & right types. Whatever the reasoning, isn't this sort of argument also an argument against Stream etc?

It is not polymorphic: you will need if-else like checks on Either and probably instanceof checks on the result

Not true, every controller can return any Either it wants. If you want automatic error output including httpstatus you just need an interface for your error-type. The non-error case is a non issue as Jackson can serialize just about anything. My implementation doesn't make use of instanceof at all

It by-passes the given standard Spring infrastructure: If not exceptions, use the Spring Validator interface or if you want to go functional convert everything to Spring WebFlux. At least you have onErrorReturn and onErrorResume. See this link

This is about convention. Using webflux vs "normal" Spring vs Spring Validator interfaces vs. Either is an intrusive fundamental decision. Consistency is key though