OK, I'll bite: "What's wrong with RPi?"
* Expensive
* Physically large
* Sucks power like crazy
* Slow
* Hardware is unreliable and inconsistent
* Constant hardware availability issues
* Form factor changes on every release
* CPU changes on every release
* I/O is adequate if you're building a PC replacement. It doesn't have the right sort of I/O for embedded tasks (multiple SPI buses, multiple serial buses and so on).
The RPi is a fine learning tool, but don't treat it as production-ready. I strongly recommend the BeagleBone Black instead.