Hi, @iluetkeb.
We chose TTL, RS485 communication to use relatively inexpensive hardware. And since DYNAMIXEL, one of the actuators, uses serial interface and protocol is open, we applied it.
But finally, we plan to make other communications available through the abstraction layer.
We are planning Modbus as the second protocol to be implemented.
The current messages only support some standard messages, and there is a need to worry about adding and customizing messages.
It is still in its infancy, so there is room for improvement. 
In addition, we are using ROS2 in TB3. The ROS2 related code (including microRTPS) used in TB3 is the same as XEL Network.
We want to contribute to bare-metal support.
Specifically, we plan to implement a library that can be used in Arduino(like rosserial_arduino).
The project name is ros2arduino(tentative).
Currently, the target board is MEGA2560 (8bit) and ZERO (32bit).
Of course! It would be nice if we had time to talk together.