"Weaknesses" of the SD(P) classes
One, although a SD(P) is larger than a normal SD it is structurally weaker because of the hollow core. In practical terms this means that it is easier to destroy. Two, an SD(P) is limited to no more than 20 min of missile combat at rapid fire (broadside every 12 sec). In practical terms they run out of missiles very quickly. Three, the energy armament of a SD(P) is extremely weak (my best guess is that it is no more than that of a BB). What that means is that if a SD(P) gets into energy combat with a regular SD it will be turned into many small pieces.
(1) The relative structural weakness of the SD(P) should not be overemphasized. An SD(P) with a hollow core is probably at least as tough as a DN with a solid core. In short, while there is some loss of structural strength, it is not truly significant unless you can somehow get the warhead to explode inside the ship.
(2) An SD(P) is limited to 20 minutes of rapid missile fire. If fire rates are slowed, endurance increases. Moreover, if you have a target that can stand up to 20 minutes of the massed fire a wall of SD(P)s can hand out, then you shouldn't have pissed off the Invincible Armada of Ming the Merciless (or Skylark DuQuesne) to start with.
(3) The energy armament of an SD(P) is not significantly weaker than that of a conventional SD. There are no standard broadside missile launchers, which frees up space for an energy armament which is very nearly as strong as a standard SD(P)'s plus additional point defense. [David has admitted that he was thinking ahead of himself when he wrote this line about not having broadside missile batteries. He was thinking of the newer RMN Invictus-class, not the older Harrington/Medusa-class SD(P). -Ed.]
I am not saying that the SD(P) doesn't have weaknesses, and -- trust me! -- no single ship type is ever going to be the God Weapon to end all God Weapons (well, except maybe a 5th Empire planetoid), but as I say, it would be a serious error to overestimate the SD(P)s flaws.