One of the things that tends to cause me to get the most frustrated in Hearthstone is some of the cards that have incredible rng swings.
I don't mind things like rag going for face when you have 8 hp left and a full board. Obviously losing that way is not particularly fun, but to me feels normal.
What doesn't to me is things like shredder popping a doomsayer or an unstable portal giving a Mage Varian. To me, that's just too much RNG. It feels far too much outside "normal" card rng limits.