I2C? pshh! Who needs I2C when you got CAN! But yeah, by far the worst thing I find with the 33F is that I need to level convert everything to communicate with 5V. Even worse is reducing 5V analog signals to be accepted by the 33F's 3.3V ADC, particularily when you need very very precise measurements like integrating gyroscope or accelerometers readings. It kind of really blows when you consider the changing offsets for the op-amp buffers and varying resistance of the resistive dividers, it's enough to add tens of mV of error which is ginormous when you are integrating the thing.
Why no 5V 33F?! Why? Why?! WHY?! Either that or 3V everything else which is a bit of a problem for ring gyros right now