While your question is vague or open to interpretation ... here's my opinion:
First of all my definition of generic components are those with out any product name (i.e. Ultegra) or even brand name (i.e. Shimano)
They might be "considered" inferior in part due to effective marketing - or just plain hype. Or they might be considered inferior due to negative stigma, since in some unrelated products, no name brand products to tend to be built to cheaper materials that aren't meant to last as long BUT you shouldn't assume this.
I know that a few years ago some of the tires sold in the US were consider generic and thus inferior but it turned out they were just as good. The other thing to mention is a lot of these generic products are made at the same plant as the brand name product - they're known as "private label" (doesn't that sound so much nicer?, thats marketing!)
Bottom line, you really need to look at each instance individually. Maybe check with others who have the specific part.