That's a shame wowlife.
I love my Dell XPS 12 - easily the best machine I've ever used and I find the engineering to be top notch.
I experience none of the problems you've outlined so it seems like there is something seriously wrong with yours!
Pretty crazy to get all those components replaced instead of getting given an entirely new machine. Was that yours or Dell's decision?
The brightness problems can probably be fixed in your settings - though i'm sure you've Googled and tried all of this:
http://www.howtogeek.com/107173/disable-windows-8s-adaptive-brightness-to-fix-dark-screen-problems/
If none of that works then yes, there is a fault. But that is exclusive to your broken machine and not representative of all Dell XPS12 machines.
Cheers,
Peter