The flaw about comparing Nightfall and Empathic Bond is that you're comparing damage vs heal, which is balanced in a very different way. Sure, you might see damage being the same as healing considering a single target, but remember that damage in itself helps you attain a win condition (bringing the enemy's HP down to 0 obviously) compared to healing which still requires a second win condition (either defeating your enemy with damage, or decking out). Take for example, the Heal card. A healing of 20 HP doesn't necessarily mean you get an overall 20% advantage over winning the game, but say, a card that would directly deal 20 damage should be a lot more expensive. Drain life and ice bolt both wouldn't deal over 10 damage unless if it's on a large stall, and fire bolt rarely over 15. But since it's arguable that these cards also double as creature control, we can only compare Unstable Gas to it, which is much more expensive and slower than Heal is, but is still considered as balanced, or at least not really underpowered.
So if you look at it closely:
6x23 healing wouldn't guarantee you a win, you can still get stalled.
6x23 damage is pretty much a guaranteed win.
