I too am more of a fan of the Swagger/OpenAPI approach (in general terms), but I find it valuable to be able to share validation logic too. (Unless your validation requirements are extremely simple.)
Of course Swagger/OpenAPI doesn't preclude using the same language on the client and server side, so one could always combine "isomorphic" approach with the "service definition" approach.
Of course Swagger/OpenAPI doesn't preclude using the same language on the client and server side, so one could always combine "isomorphic" approach with the "service definition" approach.