Shopped at both for decades. Bought my first pressure washer from Northern in 1990. My bro has it and it still runs great.
Both stores buy from the same factories in China. Both stores buy from outlets other than China. However, Harbor Freight, faced with higher prices, has demanded better quality, which the Chinese can produce. That is where their in-house "made-up" name brand items have come from. Hard to believe but, their low end tools have gotten even worse.
I use a half dozen grinders when I weld. It takes too long to swap between a grinding wheel, a flap disk, a cut-off wheel, or a wire wheel. Two years ago they came out with a killer lightweight grinder you could routinely pick up for $14 on sale. I have only had one of these get returned and it was a brand new one. Seems the arbor nuts were all mis-machined and the wheel wobbled and vibrated. At the urging of a friend who was seriously injured by a HF grinder a decade back, I have now bought re-man Makita grinders for under $40 (CPO Outlets) and they are very nice. I still have three or four original HF grinders from 20 years ago and they still work fine. I will swap into the Makita tools as I do more projects.
I was about to place an order from Northern for some brush grubbing tools but they had them on Amazon for a little less with free shipping. It all added up to about 80 lbs of stuff and Northern would have killed me on shipping. I bought a Polar trailer from them that I love and shipping was close to sixty bucks because it was oversize.
I like to watch that crazy Canuck on YouTube, AvE. He tears into the innards of many items today and has an engineering background that helps you understand what things are made of and what the industry standard is. You have to be able to handle a "little" salty language but his information is really good. (FYI: Skookem Jim was an Indian miner/guide in the NW territories that basically started the gold rush when he showed up in San Francisco with a couple of bags of gold. LOL... You'll hear references to "That Skookem." He's pretty funny.)
This one is on the newer HF power tools. These tools are certainly better than the HF tools of old but AvE shows you
how much better in this episode. My experience is that it's better to buy a re-man name brand tool for the same price and get more robust construction. I have been buying re-man power tools for decades and have only had one tool (Bosch) with an issue. Still have it after a decade and it still runs awesomely after months of abuse by subs during my remodel. (Why don't subs have their own tools? BECAUSE THEY BREAK THEIR CHEAP TOOLS!!!)
https://www.youtube.com/watch?v=3ZfYF9aIO7U