I don't use save or die effects. Maybe I would if I allowed raising the dead.
I also find it bad design that save or die effects increase in number as levels go up, IMO characters are supposed to get tougher and harder to kill with levels. In effect the situation ends up the other way around in D&D. You are more likely to die instantly at high levels than at middle levels.
Poison is inbetween. I'm glad it's not save or die, but I think most of them are far too weak, and I hate that poison strength has been coupled with creature size. In fantasy as well as real life deadly, deadly poisons come attached to tiny creatures and I'd like the game to reflect that at least marginally.
I find that poison is actually one of the areas where the game rules have progressed the most.
Poison (or rather venom as that is a more correct term for natural biological produced poisons used by creatures) is utilized by animals for two reasons. Defense or Attack.
Defense poisons are rarely lethat because they are meant to teach the attacker a lesson. "I'm bad food material, don't hunt me again and teach your offspring not to as well".
Attack posions are usually lethal
for creatures that pose a natural prey to the attacker. It is true that there are snakes and spiders that have venoms that are far stronger than they technically need to be and can kill much larger animals than they can possibly feed on. But even when they are lethal they most often work slow.
A poison that kills or paralyses in a few seconds is not very "realistic"*. But a poison that slowly saps your strength/constitution or slowly does damage as your tissue dissolves in the area of the bite is very "realistic". I find that the new mechanics reflect this better.
To improve them all you have to do is keep letting characters roll saves until they pass one (or two, three or whatever you find reasonable) and let the poison do it's base damage each time they fail a save.
* but there are few mineral poisons that have that capacity in high doses.