I obviously haven't played with it but this is my impression from reading about it. It takes a scaling rune and makes it into more of an unreliable and overcomplicated mid-game rune while also greatly narrowing the viable users. If you want to go a tanky champion or a tankier build but still benefit from up-front brust, this becomes useless. You have to rely on your team to get your target low enough for you to have a useful rune.
Moreover, the damage will seem rather odd and unexpected. With up-front burst, we know when someone's in trouble. Here, it will seem like a champion used their damage rotation and you're "safe" only to suddenly be burst when your HP ticks a bit below 50% HP. It's harder to anticipate and plan out what you're doing. There's also anti-synergy with some of its intended users. Assassins often have abilities or passives to execute targets so this can lead to an overkill scenario late game. The difficulty is getting them to threshold below 50%, not finishing them.
It used to be a play style oriented rune and it's now just a rune for mobile burst champions. For example, let's say I want to build a beefier Ekko with DH or maybe a Kha with Sterak's or even a Kayn. It isn't as effective and doesn't meet the late game fantasy scenario. It becomes a "I sure hope, I can have this not be useless in a clutch scenario" type of rune. It isn't about efficient pathing and scaling into a reliable power curve. It just destroys everything this was supposed to be as I see it. And for what benefit? It basically becomes a delayed Electrocute with a bit better scaling. It's gimmicky and actually restricts play style/champ options. For example, split-pushing with DH is now much weaker.
The whole stacking fantasy breaks apart if you can't have a guaranteed proc. This proposal even has it with a CD (unless you get a takedown) so you can't just try to find a DH stack in a fight (perhaps a juicy cannon dying) and then go ham. Properly using the souls of dead enemies/allies/monsters/cannons etc. was also one of the best aspects of the current DH and provided skill expression.
nutshell: **absolutely hate it - takes a well designed skillful rune and turns it into something requiring no skill and having nothing interesting to offer while also totally changing its identity. **
this isn't even a good jg rune anymore because laners will be able to proc. the stacking mechanism far more frequently and will also be able to stack more AP/AD to get the most bang for the buck out of ratios.
BTW: putting more power budget into a scaling AP/AD ratio also reduces play style options and makes DH much worse if you want to adapt your build to be tankier