One way outlaws could be made more viable is to give them some niche uses. Giving them a multiplier versus native units could be one such niche use.
Outlaws everywhere menace local populations, robbing and extorting them, so a small bonus versus local units from native allies makes sense. It would also match with more specific themes like the “Cowboys and Indians” rivalry or real life examples such as the Cypress Hills massacre.
The bonus could be gated behind an upgrade called “Banditry” or “Vigilantism”. An anti-native bonus such as this would make outlaws a viable option to counter native rushes.