The off brands are not always badly built, but they are more unpredictable in quality and design than the establilshed brands (which you should understand can also be duds sometimes).
For example, I have very happily used a no-name off-brand pack for the past six years. I bought it for $45 and have taken it for at least 800 miles since then. I did modify it, mostly to remove stuff I didn't need or want, making it simpler and lighter. Also, and This Is Important: it fit me very well. There is no substitute for a good fit.
In terms of design, the off brands tend to crib their ideas from the "hot new packs" from about two years earlier. How closely they follow the design depends on how easy it is to duplicate the design and features cheaply.
The biggest question mark will be quality of materials and construction. Sometimes the need to omit features makes an off-brand pack simpler and cleaner, which is usually A Good Thing. Sometimes it just results in a needlessly complex pack made on the cheap. The lighter the pack, the less you can take shotcuts on quality materials and construction.
Just my $0.02.