Ah, this is why Del was asking me about it earlier.
Ben said it best: it's a hybrid ship and it sees a variety of uses as a result. If just left to armor, it's tougher than pretty much any GB -- most transports are. But it's also enormous compared to gunboats and uses the now-heavily scaled back transport shields.
As I told del, give it a CD and the 150 thrust speed of gunboats and maybe it'd lean toward the gunboat side of the scale. As it is, a GB will run rings around it and can leave a fight against a lone Salvager at will if things go pear-shaped. A lot of large ship combat is based in running around in EK and smashing each other on firing passes. The Salvager is at a marked disadvantage due to its inability to stop the drifting of an opponent.
Also factor in the surprisingly shorter range of gunboat turrets as compared to transport guns, and the fact that most transports that are meant to be able to hold their own in a fight often have as many as 8 to over 10 turrets that can fire in a single direction. Say your typical gunboat turret has 3000 DPS, and your average Transport turret does 1800, at a greater range. (We'll leave out pulse guns, razors and missiles for the time being despite most Salvager captains leaning toward having a couple of these onboard.)
Your salvager will, barring an opponent sitting in those very minute cones at 10 and 2 o'clock where all six turrets can fire on a target, fire a maximum of five guns. Say a Liberty Rogue in a Pirate Transport and a Salvager come to blows over some nice floating cardamine that was dumped in space by the LPI and they didn't bother to tractor or destroy it. The Pirate Transport has 8 guns that fire forward, the Salvager has five.
8*1800=14400
5*3000=15000
The Salvager also has a slight edge on regens, power supply (the transport shield drain is only an inconvenience as opposed to crippling as it is on the PTrans) and armor. The PTrans has a CD and can engine kill around the Salvager with impunity as the Salvager can't limit the transport's mobility in turn.
The end result? The fight ends up much, much closer than one would expect.