Clearly stating the limitations up front helps to avoid many arguments. It was too slow or the vendor had to customize the product to work. Trust me, whenever the vendor or software developer has to workaround a problem to make something run on your computer, the chances are good that it will never work the way either party thought it would. I do have one tip for purchasers of custom software or specialty hardware. Make sure that the specifications are clear on how the product will be tested.
Vendors usually have a test program using dummy data.
There is nothing inherently wrong with that. If the test data matches the real world then the test results are valid. In the real world however the purchaser only cares about its data. If possible, the purchaser should make a duplicate set of real data and insist on the right to test the product using the real data. I consider these two items as one in computer technology.