The Mercedes automatic's firm, hard shift is described by knowledgeable commenters as engineered-in rather than a fault, protecting the clutches by minimizing slip; properly serviced these transmissions are considered nearly bulletproof. Automatic 300s are also noted as scarce, which some see as bolstering their investment value.
Automatic gearbox: harsh shift is by design, and rare
Mercedes-Benz 300 Adenauer (W186/W189) · mechanical