It may just be a product of people only posting bad experiences whilst those with no issues are too busy using their machines to take the time to praise them but there would appear to big considerable issues with build quality in the alienware and MSI machines according to the various forums ive seen. I'm just not convinced that the quality control is where it should be with these two brands and my advice would be to look at other options. Its not unheard of to see people on their 3rd or 4th machine in some instances - whether they are super picky or plain unlucky matters little, if you're forking out £2500+ on a laptop you shouldnt expect to get too many problems.
I'll be getting an Aorus v7 most likely when they are released for what its worth.
Edit: Just thought I'd follow up with my experience of alienware laptops. I've had 3, two m14x's (r1 and r2) and a m17xr3. The m14xr1 was good until the mainboard gpu started artifacting, had a lt of use from it up til then and was ready for an upgrade so wasn't a major issue for me but perhaps should've lasted longer. The m17xr3 came with gtx580m card that failed and was a well known issue with that particular gpu, I did a replacement upgrade myself to a gtx780m and it still works but I can't get it to work on battery alone despite replacing the battery. It now runs my Kodi tv and little else. My m14xr2 has been faultless and is still my current machine when i'm away. I'm doing some more CAD and CAM so require a larger display than 14" and could really use some more gpu horsepower than the gtx765m that it is equipped with. I have no personal experience of MSI laptops, but a bit of googling will tell you a lot about their customer service.