In a longer fight it gives you a guaranteed free hit for a mana potion.
But it's not
a guaranteed free hit. It's a 50% chance
of getting a free hit at some unspecified point in the future.
one of the major problems with it is actually that you're not getting access to it during the parts of the game that it was built to be useful for
Quicksilver would be quite useful for non-rogues who have not been able to purchase Tikki Tooki's dodging boon, so making it a potion available as a preparation would be another matter entirely.
Once you have a passive chance to dodge, though, it becomes absolutely horrendous. This makes it useless as Tikki Tooki boon since he offers a permanent chance to dodge as one of his boons. Since this dodge chance costs the same amount of piety as creating a quicksilver potion and doesn't
require expenditure of a mana potion, no one in their right mind would create quicksilver before buying this boon. Since quicksilver sucks once you have a natural dodge chance, no one in their right mind would create quicksilver after buying this boon. Something needs to give; we have a completely useless boon here.