I'd guess the only challenge would be integrating them into a quest that justifies a good number of them being recruited, it's easier to do with regular subraces and completely different ones since they are:
1)tend to congregate in communities full of their people while Half-breeds are usually spread out in communities populated by either one of their parent races
2)have a justified reason for not already being apart of a faction, while half-breeds tend to stay in the faction that is closer to the values they were raised with