These drives are a PITA! But the connectors are not propriety, they are just USB. The problem is none of the hardware tools for data recovery can *easily* access the firmware on these drives using the USB interface. Although it's possible in some cases, if the heads are working fine and the problem is just firmware related (which is rare since these drives are almost always dropped), to access the firmware and fix problems using the USB port or terminal port.
The reason people use *compatible* SATA PCBs, after swapping the ROM, is to fix firmware problems more easily. However, most of these drives are also encrypted and cannot work without the original PCB, which can make things a whole lot more difficult (if the PCB itself was actually bad, or if it has bad sectors). The fact that you need to "image" the drive using a USB port means most of the special read commands that work with SATA/IDE drives will not work and it's harder to get a good image.
Anyhow, enough venting on these drives (I had 3 of them this week and only recovered 1): what was the original problem with your client's drive? Was it dropped? Did it just stop working randomly? Does it spin up? Does it make any unusual sounds? Can you hear the heads moving/reading?