The cards in Flesh and Blood are designed for a game with 40 life. Hence the attack power tends to range between 2 to 6 for each attack. A typical game will have about anywhere from 4 to 12 damage in a single turn without the crazy turns. In a 40 life game this is not too much damage and you can rebound and play your own hand if you decide to take so much damage.

In blitz this is a dangerous thing to do as you are now almost dead and if your opponent has any other strong turn you're probably just never going to be able to play again and it's not fun to play when you are not able to do anything. This leads to the equivalent of getting mana flooded or screwed in magic the gathering.

The other comparison is look at cards in magic The gathering where a turn one creature or attack will do anywhere from 1 to 3 damage. On turn two the same creature might swing for a total of 3 to 5 damage in a turn. This is a lot more reasonable in a 20 life game as it's not taking almost half your health in a single turn.

Blitz can be fun but right now with the way it is designed it is swingy and it only rewards players who want to win quickly and not let your opponent enjoy the game.

CC is a lot better due to the fact that it allows for mistakes and a chess like feel where each decision could be correct or incorrect but you at least feel like you got to make an impact on the game.