An earnout bridges a disagreement about the future, and it shifts risk to the seller. Accept one when the gap is genuinely about growth you believe in, and negotiate the measurement terms harder than the amount.
More detail
The dangerous part of an earnout is not the percentage, it is who controls the outcome. After closing, the buyer runs the business, and their decisions about pricing, overhead allocation, capital spending, and even which customers to pursue all affect the number your payment is measured against. Protective terms matter more than the headline: measure on revenue or gross profit rather than net income where possible, define the accounting method precisely, cap the overhead the buyer can allocate, specify who makes operating decisions during the period, and keep the period short. Two years is common and three is long. A seller note with a stated interest rate is frequently a better bridge than an earnout, because it does not depend on performance the seller no longer controls.