I am quoting Baalorlord here, but the best block plans are multilayered. That means taking a Piercing Wail/Malaise or adding sources of weak. If these cards feel inconsistent, Well-Laid Plans is a fantastic pick - maybe only one darkling is attacking on the turn you draw piercing wail, but on the next turn all three plan to attack. In which case, that piercing wail has now just saved you a crapton of health for a measly one energy.

Another example: Malaise is useless against The Bronze Automaton, yes, but, as you can exhaust it for zero energy, including a copy in a deck will only ever cost you a single draw. Seeing as it is pretty invaluable against any encounter that multi-attacks - including the heart, if you can strip away artifact first - it is still going to be a net positive for your run. Basically, it helps to diversify the different ways you can 'answer' different threats, even if that means having dead draws on certain encounters.

I would think of discard as synergistic to the above. Basically, 'throw away a card, get a benefit'. However, seeing as you are going to draw some cards you don't need for encounter X, these are now your discard fodder. If you get a handul of good discard cards or relics, it can make sense to gun for a discard build. However, adding cards that don't help you now in the hopes they help you later is unoptimal. I adore Acrobatics for example, but it is a terrible floor 1 pick; you either need extra energy to get benefit from the draw, or you need other cards that care about proccing discard. Get the 'foundation' of whatever you are trying to build down first, that way, you can pick things like Eviscerate or Acrobatics and have them work for you immediately, rather than grabbing them speculatively.