Most of my clients' customers already message them on WhatsApp, which makes it the obvious place to automate, and the easiest place to get wrong. On the WhatsApp Business API you cannot start a conversation without an approved template, and I have had one rejected for sounding promotional. The messages worth automating are the boring, useful ones: reminders, confirmations, follow-ups.
The rules I build by come from real projects. A bot asks two or three quick questions before a person sees a lead: what they need, when, and where. It steps aside the moment something is medical or urgent, when it fails to understand a reply twice, or when someone asks for a person. And I talked a client out of mass broadcasts, because messaging people who never asked gets a number blocked. These posts go into each of those decisions.
