Hero units, upon leveling up, can generally match the power level of ordinary units of the same tier.
A level 10 Hero Knight's overall strength is roughly equivalent to that of an Eleventh Order Knight, and weaker than a 12th Order Holy Knight.
After reaching level 10, a Knight Hero's growth in strength ceases to be just about leveling up.
Hero skills and magic abilities gradually become vital parameters in measuring a hero's strength. One must attain a certain standard in hero skills and magic before triggering a level up.
Yan Xing's hero level is stuck at level 10 because there is no pathway for learning the necessary hero skills and magic.
The Knights' Guild in Nuta City can impart Hero-level knight skills. The Mages' Guild can provide Hero-level magic.
However, the Knight Instructors in the Knights' Guild and the Mage Instructors in the Mages' Guild have limited expertise.