MorganS:
A more advanced chip, such as the ones used in the Teensy 3.x series can have variable drive strength. It is possible to configure the output pin driving capability so that you don't get rapid rises and ringing.
Good observation. I suspect the output driver switches in variable series resistances. The rule of thumb we used is to not exceed 1 V/nS for transition rates to avoid crosstalk and ringing. At light capacitive loads or heavy inductive loads you need more resistance.
I was curious so I looked at the digital output of an Uno. I put capacitors up to the value of 1000 microfarads on an output and looked at the transitions. No external series resistance. Nothing blew. I saw the same output impedance for rise and fall. Looks like 70 ohms to me. The output current limits itself to 80 mA. That is the nominal peak value the output can supply.