You couldnt really say "beat to death as mining hardware" when your creating it for that purpose. If its obeying the purpose, a 90-day warranty wouldnt be within the reasonable standard. There are many factors when coming up with a warranty, and for this product being $3000+ and being used for its intended use to break within 1 year, would most likely end in a lawsuit with your company if someone wanted to. There is an implied warranty, and you will be held accountable for implied warranties if the product cannot do what its built to do for atleast 1+ years. This is why you see long warranties in the computer sector.
There's a pretty simple solution for all the potential buyers out there. If you don't like the terms/warranty, then don't buy it, simple as that. No one is holding a gun to the buyer's head to buy a product with only 90 days warranty.
If you would like a 1 year warranty, then I suggest senseless create a 3rd product on his website for $15,000 board with included 1 year warranty. There, buyer's have options now.
Easy as 1, 2, 3.