Not really. Their strength was in numbers and surprise attacks only.
As soon as they came under fire themselves they blew up like firecrackers.
they were total glass cannons.
That's because the shields were down. Once they were launched, they'd be as tough as any Star Destroyer. Poe nearly obliterated the Resistance taking down one dreadnaught with a minuscule fraction of that firepower, and only succeeded at all because of Hux's ego (disdaining less spectacular fighter-based attacks, and attacking the empty base before the ships in orbit), and he was vindicated by history, despite his demotion for violating orders; with hyperspace tracking, if the dreadnaught had survived, they would've been picked off as soon as they dropped out of hyperspace. Two dreadnaughts at D'Qar would've ended the Resistance. Twenty Sith Destroyers in Piett's fleet at Endor instead of the same number of conventional destroyers would've wiped out the Rebellion before they even understood what was happening. Heck, one Sith Destroyer at Scarif or Yavin would've done it.
their main weapon wasn’t the giant super laser. It was fear.
You've got that right. Wouldn't the purest expression of Tarkin's dark dream be a Destroyer in orbit over any planet in the galaxy that could shatter the entire world at the first hint of defiance by anyone on it? Way more effective than one or two Death Stars that could only be in one place at a time, or even a Starkiller Base that could attack from anywhere without warning, but after a significant lead-up.
The empire built 25,000 imperial star destroyers. A canon figure.
Even with the losses at endor and jaku, that probably only counted for a few hundred. With a few thousand lost in other battles.
Thousands likely survived the imperial rements retreat into the unknown regions. Those where likely refited with the scaled down super laser later on.
That's a logical interpretation, but according to the Visual Dictionary, the Sith Destroyers were, in fact,
a different design that was almost twice the size of an ISD-1 despite looking perfectly identical, down to the relative size and placement of the windows (I guess the officers on the bridge probably got a little step-stool so they could see out the viewport).
Still, Rose's line about researching old Destroyers, the color of the Sith TIE Fighters, and the fact that the ships looked exactly like Early Empire/Extremely Late Republic-era ships suggests they're an older generation of technology. Potentially predating the Death Stars, definitely predating Starkiller Base. And making very little sense either way. J.J. Abrams had a good point about superweapon inflation leading to Starkiller Base, but going further, to the point where it's even possible to build not just one, but thousands or pocket Death Stars in perfect secrecy, and being this overt about it, leads to certain implications about the universe. How much further will it go? In a thousand years, will every fighter and smuggling freighter have a planet-cracking laser on it? Will there be a hand-held version?