Picture an account where form submit, phone click, WhatsApp click and a thank-you page view are all primary. Smart Bidding will chase whatever is cheapest, and that's almost always the page view. The campaign column then shows healthy lead volume while sales says the calls aren't any better. That's the classic symptom.
My view is that only the action tied to a real sales outcome should be primary. Everything else goes to secondary so you keep the data without it steering bids. Micro-conversions teach the algorithm to chase easy clicks, so cost per form submit looks great while cost per qualified lead gets worse.
The counter-argument is volume. If a bid strategy only sees ten primary conversions a month, it may not learn at all. How do you handle that when the real outcome takes weeks to show up?