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:

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