Recently there have been several USBi failures reported by SigmaDSP users. I have been replacing the broken boards, but in the meantime I have been looking for the root cause of the reliability problems.
We used a Cypress USB Microcontroller in the design, and the PCB schematic was based on one of their reference designs. However, that design did not have any ESD or overvoltage protection on the D+ and D- lines, which are the differential signaling lines on the USB connector. Also, there was an issue in the firmware where SPI lines were always actively driving, not going high-impedance, when I2C was active and the SPI bus was idle.
In order to address these problems and improve the reliability of the USBi, we added clamp diodes, supply protection ferrite beads, SPI line series protection resistors, and smarter firmware. These new USBi boards will be shipping starting next year.