A centrifugal pump adds energy by accelerating liquid through an impeller, then converting velocity to pressure. That works beautifully on thin liquids and progressively worse as viscosity rises, because a viscous fluid resists being accelerated and dissipates the energy as heat instead.

A positive displacement pump traps a fixed volume and moves it, so viscosity barely affects the flow rate at all. It is the answer for anything thick, and it also delivers a flow proportional to speed, which is what makes it the answer for dosing.

The trade is what happens at a closed valve. A centrifugal pump against a closed discharge simply churns; a positive displacement pump keeps trying to displace its volume and will burst something, which is why it needs a relief path that a centrifugal installation does not.

Shear is the third consideration and it cuts across both. A high speed impeller damages shear sensitive product, which is why a dairy uses a lobe rotor pump on cream and a centrifugal on water, even though both liquids are thin.