While theorycrafting is sometimes unreliable, there are some cards that are just obviously too weak to be played, especially when compared to existing cards. What if someone suggested a 7 mana 4/4 Overload 2 for the reverse memes? Would you need to playtest that card to know that it's strictly worse than existing cards that are too weak to be played? Or is it obvious that card would be useless?

Frankly, your card would be too weak even if it couldn't target your own minions. Look at [[Demolisher]], for example, a card that has basically never been worth playing in the history of Hearthstone. In exchange for only hitting minions (generally an upside, if you made it only able to hit enemies), your card loses the ability to hit damaged minions, has fewer stats, and a worse stat distribution making it almost trivial to kill before it even gets to trigger its effect.