The Unstoppable promotion doesn't appear to be functional. I'm playing using the most recent version, and I have a Knights of the Round Table, lead by a Warlord, with the Unstoppable promotion. I moved him onto a forest with a road, and attacked an enemy army in a forest on a road in my territory. Before attacking he had 7 strength and 1.5 movement, after attacking my Knights of the Round Table won the battle, ending with 7 strength and 0.5 movement. The unit did not appear to have gained a movement point, and was unable to initiate a second attack with the remaining 0.5 movement points.
EDIT:
The next turn (possibly unrelated), the game closes without an error while processing the turn. I tried reloading a save from a prior turn and continuing forward, but it always closes with no apparent error at the same turn. Attached is a save I have just before attacking with the Knights of the Round Table, and one turn before the game closes on me. This is on the "accurate_earth_232x112_blank" scenario.
EDIT2:
As a test I exported teh save game as a World Builder save, and loaded it as a new scenario. The turn processed successfully, it turns out that same turn was the turn "Enlightenment clears the minds and mystical creatures disappear from the world." So maybe that's related?