My extensive experience of being a PayPal seller is quite the contrary. People request a charge back, PayPal "encourages" them to contact you, which essentially is just an extra step in the charge back process that they can skip, then nearly every time the money is returned with nothing the seller can do about it.
Afaict it depends on the kind of transaction. For services PayPal is very buyer-friendly. The main area in which I've consistently heard of buyers unhappy that PayPal sides with the seller is with purchases of physical items. It seems like PayPal accepts any kind of proof-of-shipment as conclusive in those cases. If the seller can show a tracking code with proof of delivery, PayPal closes the dispute in favor of the seller. This ends up making it easy for sellers to ship severely subpar goods (e.g. relabeling used things as new), and even carry out outright scams like shipping an empty box full of packing peanuts, or shipping a cheap compact camera to fulfill a high-end sale.