Personally I'm not sure if Cassius's Death Curse is that much of a thing.
1) He wasn't Wizard enough to kill Harry, seems kinda silly that he can kill every SO Harry ever has.
2) Margaret's Death Curse only hung around for so long because she anchored it to her bloodline. Logically, snake boy's curse would have burned up by now since we've had a couple thousand sunrises since then.
So yeah, wouldn't be surprised if this is mostly just Harry having miserable luck.
Cassius might not be able to cast a long lasting death curse by his own power, but if it is only to aggravate an already bad spell to begin with, well, it could become something deadly.
Even if we discount Cassius's curse, Harry's birth is shrouded in many mysteries in itself. First he is starborn. I don't really know what significance his birth carry, , but the fact he is starborn makes him a target of many great powers even since his conception. Second, his mother died via an entropy curse during his birth. As we know an entropy curse is exactly a type of bad luck curse. Who know what kind of energies attached to the newly born Harry at the time. He also gotten marked by HWW behind.
What I wanted to say is this. It would not surprise me if Harry's mmisfortune turn out to be more complicated than just simple bad luck and Cassius's *die alone* curse just added to it. There sure as heck enough background reason should Butcher choose to tell the story this way.