You have a 25% chance per writ to get a survey. If the survey is upper Craglorn and either blacksmithing, woodwork or clothing, you have a 1.25% per node to get a nirncrux. Harvesting a normal upper Craglorn node has a 0.25% per node to get a nirncrux. The bug where Upper Craglorn Blacksmithing nodes had a 100% chance of giving nirncrux has been fixed.
The dev's have already posted feedback that they know the crafting writs are not really worth for thoses who have maxed crafting, and they are looking at adding 'master writs' with different rewards