Both the SPI & I2C implementations are seriously crippled and, therefore, useless in many standard applications:
https://forum.sierrawireless.com/t/i2c-bus-how-to-send-a-repeated-start-condition/4931/3
https://forum.sierrawireless.com/t/can-open-at-do-full-duplex-spi-yet/4516/5
https://forum.sierrawireless.com/t/i2c-and-spi-slave-modes/4932/1
