On not trusting the backend... I think the bigger issue is not having an integrated process to generate the API client with CI/CD, or otherwise the separation of Back-End and Front-End development. Even if they are separate teams, you still may want to share a repository for the front ends and back-ends... even if using differng technology.

If you cannot keep your releases or codebase in sync, then I generally suggest the UI has its' own API service that is tightly coupled to the front end and deal with variance from that coupled API service. This is even more beneficial if your backend services are in a different language.

I do find that API specifications and API client generation is helpful. It can work exceedingly well for .Net backends, as an example, where it's a simple matter of decorators for parameter and response types.