I have several diamond files, and diamond stones for that matter! There are a mix of brands, DMT, 3M, Vallorbe and they have all been nothing short of superb! the common factor, of course, is that they are all made in the U.S.A. Switzerland or somewhere else acceptable!
I have seen a few Am -Tech tools around, and they are the usual Chinese / Taiwanese shite masquerading as American made. They wouldn't fool anyone tool savvy, but I guess they fool enough!
To the O.P. Don't despair - real diamond files work well and last better, but they are VERY expensive. Now you know why so many experienced tool users place so much emphasis on Country of origin!
Depending on what you are filing, you maybe better off with a decent quality conventional file than a diamond. I only use the latter if I'm working something really hard! Whatever you get, choose something made in the western world and you will be fine!