Kind'a depends what you mean by "fully compatible"Is this board fully compatible with the drivers and other software for the Arduino IDE 1.6?
You can write sketches and upload to this board from the IDE 1.6, however the STM32 core does not support absolutely all the features in the AVR core e.g. I2C slave is not currently supported.
Nor will some existing AVR sketches work, especially those which directly access the AVR hardware.
Additionally, the STM32 is a 32 bit processor and AVR is 8 / 16 bit, so the compiler is different and also some of the built in types e.g. double is not the same as double on AVR (nor is the int data type).
Also in general don't expect AVR libraries to just work. Some AVR libs specify they are AVR only, but just need the meta data to be changed, however some libs need to be reworked a lot to get them to work on a 32bit ARM environment.