To be fair, that $40 cheap Chinese grinder at harbor freight is a major POS compared to $80 Chinese grinder at Home Depot. Not everything made is China is a major piece of ****. (My main business is dealing with Chinese manufactures.) They are willing to build something to price point that really drops quality and/or usability to the point you have to wonder if it's the same item any more.
Grinders are actually a great example. I have a couple 4.5" DeWalt grinders. Not wanting to spend $80-$100ea to get a couple more to run quick strip discs I bought the 2 "top of the line," 4.5" grinders with the paddle switch from HF, I might have paid $25-$30ea for them after discounts. They are only 6amp, and I think that is an optimistic rating at that. You can also feel vibration in the gearboxes like the machining is probably subpar.
Pretty much anything that requires machining during manufacturing is going to be garbage from harbor freight. In China, some factories use state of the art CNC machines with the operators just overseeing the programs, most factories use quality manual machines with trained operators actually running them... then places that make tools for Harbor Freight and the like, use old clapped out hand-me-down machines with untrained operators.
Harbor Freight runs bids for almost everything and goes with the cheapest manufacture. They have very little quality control or process oversight. Factories that build, what a lot of people assume are cheap tools like Ryobi, are actually owned and ran by Techtronic Industries, who own dozens of factories dedicated to building tools, they engineer everything from the ground up, and implement all the proper processes and quality control, because ultimately, they are putting a brand name they own on the tool. They are able to build actual assembly lines suited to building specific tools.
Factories that build stuff for Harbor Freight and other similar outlets can't do that, because one day they might be building grinders, the next they are building steam cleaners, the next they are assembling box fans, then maybe some other random power tool.
Not to say that TTI is the end all be all when it comes to manufacturing, but when you compare their products to harbor freight's. It's like comparing a cadillac to a original vw beetle.