The Outlaw Star is really awesome... but has no-where near the firepower or defensive capabilities.
The ship is a peice of junk in comparison to Enterprise. They hardly have the funds to keep the thing running which is one reason they keep doing odd jobs
They can barely keep it stocked, but that's because it's an advanced prototype and they're a small-time operation.
Still, it's outmatched because it's small, Outlaw Star-verse doesn't have shields to speak of, missiles with chemical explosives primary weapon, large-caliber guns work too, and using maneuverability and robot arms to rip apart enemy ships is something that works, but won't get past Trek shields.
They could pound on the E-E all day and not get through, trek ships trade torpedoes full of anti-matter.
Now, in terms of personal combat, they're a lot more capable than trek, but not in space.