Bashir was pretty good at this reversal-of-death stuff most of the time, but he appeared to drop it at least twice. He was shown capable of treating knife wounds to the lower back a couple of times, but failed to save Armin Maariza in "Duet". And he was shown capable of treating phaser burns to the torso, too, but failed to save Li Nalas in "The Siege". While it could be argued that Li got hit in a particularly bad spot, and that phaser hits usually are fatal when intended to be, the knife wound was a simple repeat of incidents where Bashir had allowed a main character to survive just fine.
Then again, Odo, too, was lax in his duties in protecting Maariza. Perhaps it was by mutual agreement of our heroes that Maariza was allowed to die in the hands of a Bajoran, considering that this had been his fondest wish for such a long time?
In contrast, I'm not too worried about the death of Jadzia: it was magic, so it was probably difficult to cure - but Dukat didn't understand or control that magic particularly well, so even if he wanted to be 100% lethal, he might plausibly fail, and leave the symbiont alive. And while crazed and all, he need not have been interested in killing the person who blocked his way - just disposing of her/it in an expedient manner
Timo Saloniemi