A lot of lead gen accounts mark a form fill, a phone click, a newsletter signup and a pricing page visit all as primary conversions. Smart Bidding then treats them as roughly equal, and the cheapest action tends to soak up the budget. A newsletter signup at a few dollars isn't worth the same as a booked consultation at a much higher cost, but the algorithm only knows that if you tell it, either through conversion values or by keeping the low-value actions out of the bidding signal.
My view is to mark only the actions you'd happily pay for as primary and move everything else to secondary. Secondary actions still show up in reports, so you can see the funnel, but they don't steer bids. The trade-off is less conversion volume for the algorithm to learn from, which hurts most on small budgets. Has anyone run it the other way and found the extra volume made up for the noise?