The choice of CPU is related to performance needs (e.g. DSP) and TDP, especially if you use a LPS or a fanless chassis. ROCK compatibility, if I’m not mistaken, is more concerned with drivers especially Ethernet.
More on LPS: If one is intending to use a 36W LPS, then a 28W TDP is too high just for the CPU, since there are other things that need the power.