The exact percentage rate of getting a card in any chest has already been mathed out from the folks who data mined the loot tables and it's very tiny (.01% iirc). All that matters is how many chests one loots (which in turn depends on how much one plays & how one plays). Some folks will have better RNG than others, but the math is defined.
holy moly. So, you are telling me there is a chance!!! I read ya, i read ya.
In all seriousness, we should not be actively farming these. If it take 10k chest pulls to get 1 (on average), just farm the gear again and re-curse it. lol. what this system does is say:
"hey, you want this new shiny don't you? well, how about I give you something you don't want...over, and over, and over again...while you farm chests to pull the same loot to rinse and repeat the curse that you don't want....over and over, and over...."
Its another way they "getcha" , by giving hope..but lets be honest, none of these curses are game breaking...and what happens if you do get a cleanser? you use it to get rid of something, and then re-curse with a 1-1000 chance you get what you actually want. lol
fun times