I'm actually not quite sure about this...
... but i doubt that all card manufacturers would use the same brand of capacitors for their card... and if they don't, well that would explain the huge differences that you see in reliability and longevity right there

Otherwise, even with a reference design, you could still easily have huge differences with heat compound, fan manufacturers and most importantly
manufacturing PROCESS .
.... someone actually has to put these cards together. And it doesn't matter if it's actually done by a human or fully/mostly automated by a machine... the quality of the manufacturing process will have a huge impact on the reliability of the final product.
The difference between ATI and Nvidia, frankly, is less important to me these days than the difference between card manufacturers.
I've had plenty of both Nvidia AND ATI cards that worked well over the years. (And as far as problems go i had my share with either company as well... the impression that stuck was, that one better be careful with new Nvidia products when their market position is too strong at that moment (the mobile GPU desaster with mass failing of inferior components comes to mind, as well as the rather infuriating memory/stutter bug in several cards of the 8800 gts series) and on the other hand.... that ATI is prone to push too hard for performance when pressed (which - for example - resulted in the rather grotesque heat/power/noise monster cards in the X2x000 series ... )
Lately tho, we ve seen very nice and comparable offerings from both companies with Nvidias Fermi and ATIs 4x000 and 5x000 series.
... in any case... Nvidia or Ati isn't really the issue in my eyes; In the past both companys have had really nice and really crappy cards (Geforce 5x000 er series that's you for example lol)
however, more importantly, I've never had a heatsink part just fall off an EVGA or Sapphire card inside the case while playing a game... (I'm looking at you PNY LOL - and that was 3 weeks after purchase mind you.).