Darkness Points are a mechanic I think looks great in principle, but requires special care by the GM.
You have to use them when you have them, and you should use them in sensible ways to make them feel like a meaningful threat to players. But you also don't want to use them too efficiently and save them up for moments where you can really hurt the players the most. In other game, when the dice fall in an attack against a PC and it's two 20s, that's bad luck for the player. The GM simply decided to make a normal attack, nothing more. But when you use Darkness Points to make an attack that was already a hit into something that kills a PC, or to trip up a PC just before it reaches safety, that's situations where you deliberately decided to kick a player in the shin when there was no need to do so.
I think it's probably a good idea to use Darkness Points early and don't hoard them in large numbers. If you have a couple, keep using single points now and then even in situations where it won't be a big dramatic game changer. Or use them in situations where the players have already gained the upper hand and are at a clear advantage, and using a few points probably won't change the eventual outcome of the scene, only make it a bit harder to get there. Another thing I could see work well is to use the points to help a villain get away, rather than letting the villain hurt the PCs. It will still suck, but extending the long term conflict a bit longer does not hurt in the way of a character getting maimed or killed.