Fitment is the product data
Buyers search by make, model and year. If your titles and attributes do not carry fitment, you match the wrong searches and pay for clicks that end in returns.
Returns feed back into everything
Compatibility errors are the main source of returns in this niche, and returns hit your real ROAS twice — you paid for the click and you refunded the order.
Where it works well
Specific, searchable parts with clear compatibility and a page that states it plainly. Vague universal-fit products struggle on both conversion and policy.