What issues will I run into when we try and do both API and UI implementations and validation on the same user story?
Some issues you will run into are:
- co-ordination of who tests what
- determining who tests what parts of security
- determining the feedback mechanisms for each
- changes in one area that affect the other
- co-coordinating validations
- standards for where validation is done (can be both)
Essentially the issues are mostly about
Communication and co-ordination
combining API and UI
Since a user story describes a feature and features could consist of UI and API changes, I guess this is pretty natural and happens all the time on different teams.
API and UI implementations and validation
Validations are something that usually have to be implemented, both on frontend (html, JS) and again on backend because we can't trust data coming from the outside. So just like in the previous answer, this is a part of a user story and so it should be implemented.
Perhaps you could give us an example of some of your concerns regarding this very topic. I don't know if I really understood your questions, it feels like you meant something concrete but expressed it in broad terms.