Unfortunately, the psu quality is often the first thing to go when vendors assemble many of those generic barebones systems- price cutting is often paramount. This is just my own experience, there may be vendors who do it differently.
The best method is to know exactly what you are buying, generic bundles have generic descriptions for a reason...On the other hand, if you can find a bundle that exactly specifies each component and is what you want, go for it, you'll probably save a few bucks.
Occasionally, you will be pleasantly surprised. I am currently assembling a new system with a motherboard bundle from Mwave- customer specifies board, processor and memory, they include a "generic" HSF. That HSF turned out to be a AVC Mega-Cool, obviously a good unit, extremely similar to the Taisol that I bought separately from a different vendor.....I probably would have known that and saved a few more bucks had I gone to the trouble of contacting Mwave about it.
If you have questions about anything, contact the potential supplier, if you don't get straight answers, go elsewhere...