There is too much options [0] and it's more flexible than it should be. I know , it sounds weird but with bootstrap all you have is 12 columns and when you have less options it's easy to plan.
Maybe the other problem is : Units are not in multiplies of 5 and that forces you unnecessarily to think. If it would be in fixed units like bootstrap or in multiplies of 5 ( pure-u-3-25 etc.) i would be less thinking while deciding the length of some div.
The idea is that you can combine columns of different sizes. If you want a column of 1/3 of the width and another of 2/3, then you just need to add pure-u-1-3 and pure-u-2-3. Maybe you're right and the "u" is unnecessary.
Maybe the other problem is : Units are not in multiplies of 5 and that forces you unnecessarily to think. If it would be in fixed units like bootstrap or in multiplies of 5 ( pure-u-3-25 etc.) i would be less thinking while deciding the length of some div.
[0] : http://yuilibrary.com/yui/docs/cssgrids/#unit-sizes