Bounty is heavily based on the faction that is boosted, every faction you will see in the update notes or your inbox which faction is boosted. For example this month had Marlowe boosted, she a mech and will do the most damage compared to heroes that aren’t boosted due to the damage multiplier you get. I would suggest working on heroes in the boosted factions when they come around so you’re always upgrading heroes in different factions,
Update notes: April 2020 Update Notes - The Marlowe Update