"Все, что нас не убивает - делает сильнее"
I admit that которое instead of что would sound weird and wouldn't make any meaning.
In this case I suppose that что gives a more abstract and general meaning without indicating a specific object associated with Все.
But in an expression: "Все вещи, которые нас не убивают - делает сильнее", I think that которые is one-way.
By the way "-" as part of a sentence is extremely weird.