Harbor Freight just varies a lot in quality by product category.
That's one of the reasons I like Harbor Freight so much. If you need, say, a reciprocating saw, but doubt you're going to use it often, you can pick one up there that will work decently for the odd job and won't put much of a dent in your wallet. Or you can invest a few more dollars and pick up one of their house brand products (Chicago Electric I think?), which will do a good job for an amateur handyman for a long time, and still won't cost a bundle. And I have a friend who's a contractor, knows exactly what he's looking for in tools, and he shops there too, because he knows what he wants and needs, and he does find stuff that meets his standards. If you want to invest in a really good tool and you don't really know tools inside and out, Harbor Freight may not be the best place to shop. But they do cover the spectrum from very cheap to pretty high quality fairly well.
Power cables are no different. If you know what you're looking for, you can find a good cable for a good price, and being an EE, I'm convinced that the Monoprice 14AWG cables are good quality for a very good price. I haven't looked at Harbor Freight's cables or extension cords but I'd bet they run pretty much the same way, you can get a very cheap one or a pretty good quality one.