If your product is used by humans, then it needs to be tested by humans - this cannot be automated. Those humans can be your QA people, or your customers. Perhaps your customers are happy to be testers, perhaps not.

Unit tests are very expensive and return little value. Conversely, a (manual?) 'smoke test' is very cheap and returns great value - the first thing you do when updating a server for example is to check it still responds (and nothing has gone wrong in the deployment process), takes 2 seconds to do, prevents highly embarrassing downtime due to a misconfigured docker pull or whatever.