Can you absolutely prove that the motherboard failure affecting the video card had
absolutely nothing to do with something else in the system, or the way the video card
was installed, or something else the user may have done tweaking the system?
If yes, then you may have a case to try and convince the motherboard manufacturer to your side.
But in the real world, unless you want to send them the whole system as is for them to
examine and determine what may have caused the failure in the video card... then there
is always going to be a reasonable doubt in their minds as to the cause of the problem.
And they will naturally be biased against taking the blame, since they don't have hundreds
of reports of the same thing happenning to other users.
The problem is that for every person that may have a legitimate complaint, there are hundreds of
users who messed up the part thru their own actions (deliberate or accidental) and are trying to
get the manufacturer or vendor to take the hit for the cost of a replacement part.