I find it hard to figure out what decks are good since the pool of decks I play against are heavily biased towards what I already think is good. For instance, I think minions are generally bad investments unless they're dragons being cheated out. Most of my decks avoid making any non-mirage minions and instead use squeezers and spells to control the board. Going by my experience iron succubus might be the best at 35 life. Is that true? Probably not, but I haven't managed to build a deck that goes wide with creatures that I like, so it might as well be true. It's hard to solve a meta in a box.
Guardians are super good what do you mean?
It's how you protect yourself against enemy attacks.
Iron and all demonic squeezers are better at high life. They all do 2 damage to you minimum. It's supposed to be their drawback.
2 damage(MINIMUM) means you die 2 times faster and kill the enemy 2 times faster.
If a game with normal squeezers would've lasted... let's say 10 turns letting the opponent draw 10 times, and giving them comeback opportunities, the same game with demonic squeezers would only last 5 turns. 2 damage minimum is a big fucking deal and the dev didn't balance it right while overusing it.
Unless you play at high health and cheat the game avoiding most systems by tanking all the damage, you'll realize that guardians are extremely useful in decks. In fact they're so useful, that loading up on spells is basically suicide. You have to sacrifice more spells during deckbuilding than guardians.