Volume is the constraint
Smart Bidding needs conversions to learn from. A product selling a handful of units a month does not provide them, so manual control and longer evaluation periods are not old-fashioned — they are correct.
Attribution windows
If the decision takes three weeks and your window is seven days, the campaigns that started the sale are credited with nothing. You will then cut them.
Micro-conversions, carefully
Quote requests, brochure downloads and configurator completions can give the bidding something to learn from — but only if they genuinely correlate with sales. Optimise toward a micro-conversion that does not, and you will buy a great deal of it.