Audio and Control Interfaces
While most processors are equipped with a range of built-in easy-to-use standard communication peripherals, FPGAs are seriously disadvantaged in this discipline. All we have is a bunch of programmable logic gates. There are cores available online for most of the common protocols, some of them even for free or some sort of free-to-use license. But realistically, they are rarely straight-forward and easy to use. Sometimes just trying to understand how to instantiate and interface the core to the rest of your system takes more time than writing and testing one yourself. Most of the times, the fully featured communication cores are too complicated anyway. We tend to use simplified and highly efficient low level communication and codec cores, which are tailored for specific tasks and easily reusable.
Communication and control: SPI, I2C, UART, RC5
Audio: I2S, SPDIF, AES/EBU, TDM
Apart from direct FPGA implementation of the protocols above, we have worked with USB, HDMI, DVI, Ethernet and others using off-the-shelf silicon for interfacing and protocol conversion.