In real life, broken bones are obviously more serious.
But in the world of anime, particularly one piece, scar is a lot more significant than broken bones.
1) Scar is a permanent remainder of an encounter or a relationship with another individual.
Think: luffy's scar by akainu, kaido's scar by oden, zoro's scar by mihawk, shank's scar by BB.
2) Broken bones can be healed easily in OP with no lasting consequences. Scar remains and stays, and it always ties back to the person who left the scar upon.