It looks like generic Arduino code needs a workaround to compile on Teensy because of this simple issue: instead of concat(), Teensy has append(). (?) This difference makes it harder for Teensy users to use Arduino Unit:

https://github.com/mmurdoch/arduinounit/issues/87

Code:
#ifdef TEENSYDUINO
  append((const char *)buffer, size);
#else
  concat((const char *)buffer, size);
#endif
I'm very new to Teensy and the C++ ecosystem, but it like a very easy thing to fix with a large payoff. ?