Auto parts store pumps have been about a 60% success rate for me over a 13 year career. Worst one was a pump on a Chevy duramax. You have to use a special tool to press pulley off and on to remove the pump body while bent over a fender or radiator support with arms fully extended. Replacement pump worked worse than the one I removed. Did that job twice back to back.

Another thing I've seen (or did once as a total novice tech) was I left the shipping plug on one of the ports of the replacement pump and installed it... Of course no fluid flow means no power steering. And when I removed what I thought was a defective pump I saw the plug and felt like a moron. System worked great with the plug removed...

My rule of thumb: if the steering works poorly turning only one direction it's the rack/steering gear box. If steering works poorly in BOTH directions, it's the pump. Only real way to test is a high pressure hydraulic gauge T'd in. And I've never seen one in real life.