Blessed potions of polymorph, when quaffed, grant you immediate polymorph control over just that one polymorph either 50% or 100% of the time. To compensate, cursed potions of polymorph will ignore changing into a dragon and will try several times to roll a monster that will make you break out of your armor. Or the cursed potion could have a wildly variable duration, so you have no idea when it will end and cannot rely on it.