Each unit has a different skill and keyword. I ran out of time to add the proper descriptions for the keywords but they are as follows:
Spearman hits on 3+ and moves faster than the other units.
Knight hits on 5+ and has a chance to block successful hits.
Squire hits on 6s and gives units within a radius of 3 +1 to hit.
The Doctor hits on 5+ and has a chance to block a unit within 3 of dying.
Adventuer hits on 4+ and runs away after attacking.
The units certainly aren't as balanced as I'd like them to be and I'd say you are very correct in saying spearman is the best unit.
I am glad you enjoyed it though, and congratulations on beating the game!