whats the actual downside of maximise conversions with no target

There's a real failure mode and you haven't hit it because of something specific about your setup. Maximise conversions with no target spends your entire budget every day and buys the most conversions it can for that money. It has no opinion about what a conversion should cost. So if cheap conversions dry up, it buys expensive ones instead - because the alternative is not spending the budget, and not spending the budget is the one thing it will not do. That is fine while your budget is the binding constraint and the traffic is fairly uniform, which is your situation. It goes wrong in two cases: Your budget rises. More money means reaching further down the quality curve, and CPA climbs with no floor to stop it. Your traffic mix changes seasonally. A quiet month means the same budget chasing worse inventory at whatever price it takes. A target doesn't make it smarter, it gives it permission to underspend rather than buy something bad. That is the whole difference. If you are happy with the cost per lead and the budget has not moved in eight months, you have not been wrong. Set a target before you increase the budget, not after.