Well he does have the stolen vessels. Such as the Centurion and Interdictors, but that really doesn't matter when you have a good chunk of your navy being categorized as cannon fodder. Which is essentially what the Derriphan has become.
During Naga Sadow's time, the Derriphan was akin to the Star Destroyer. But now that technology has advanced so far, it can barely fulfill the role of light frigate. It's really not that good anymore.
Of course back then it would be, but way back when the ships weren't decked out with tons of firepower and having all the bells and whistles. I mean the Derriphan has autoblasters...no, not turbolasers...AUTOBLASTERS which are ancient in design, and not too accurate either.