I have 2 of these units in my rack, one of them contains a Mac Mini and a MOTU Ultralite MK3 soundcard (which is actually using a rack ear, so the soundcard isn't actually attached to this adapter), and the other contains a Virus TI Snow. For both the Mac Mini and the TI Snow, I used a marker to carefully mark holes and then drilled into the undersides of their cases. Both are very securely mounted, and this adapter worked pretty well for these tasks.
As others have noted, it would be nice if this adapter contained holes in the pattern of VESA 75/100 rather than just randomly placed. For my use-case, this wasn't a big concern, since I was drilling into stuff anyways, but for others, they may be disappointed that the holes aren't aligned to the VESA standard.